How to Define Mass Shootings: Potential Policy Implications
November 19, 2024
Congressional Research Service
https://crsreports.congress.gov
R48276
Congressional Research Service
https://crsreports.congress.gov
R48276
Congressional Research Service
There is no single, widely accepted definition of a mass shooting. Instead, there is a group of numerous, disparate, and uncoordinated definitions of a mass shooting. Various entities, however, have offered related definitions. For instance, the Federal Bureau of Investigation (FBI) defines “active shooter incidents,” and since 2000 has reported annually on active shooter incidents that meet “the federal definition of ‘mass killing’.” These are the closest concepts to mass shootings offered by federal law enforcement and require consideration of the status of the incident when it becomes known to law enforcement, the context in which the shooting occurs, and the number of fatalities. Active shooter incidents are those where “individuals [are] actively engaged in killing or attempting to kill people in a confined and populated area” involving the use of a firearm where the incident is ongoing and thus has “the potential for a response to affect the outcome.” Active shooter incidents that meet the federal standard for mass killings have at least three fatalities in a single incident, not including the shooter. If the police are notified of the incident after it has concluded or the event occurs in a sparsely populated public place, then it is not considered by the FBI to be an active shooter incident. Incidents resulting from self- defense, gang violence, drug violence, domestic or residential disputes, hostage situations, or crossfire as a byproduct of another ongoing criminal act, as well as acts that do not put other people in peril, are also not considered active shooter incidents. In its report Active Shooter Incidents in the United States in 2023, the FBI identified a total of 40 active shooter incidents that resulted in a mass killing between 2021 and 2023. In contrast to the FBI’s definition, nongovernment organizations have defined mass shootings in various ways. For example, the nonprofit organization Gun Violence Archive (GVA), which attempts to track every reported incident of gun violence in the United States using publicly available sources, defines a “mass shooting” as an incident where there is “a minimum of four victims shot, not including any shooter” (the definition is agnostic to the number of fatalities). Using this definition, the GVA counts 1,965 separate mass shootings during the same period examined by the most recent FBI report (2023) on active shooter incidents.
The arguments given about what should and should not be included as requirements to define incidents of firearm violence as a mass shooting involve three primary questions:
• Should there be a minimum number of fatalities or casualties, and if so, what number should be used?
• Is the shooting itself an end goal for the perpetrator or does it occur within the context of a broader crime, such as armed robbery or drug trafficking?
• Should incidents involving domestic violence and familicide be considered mass shootings, especially if the incidents occur in a perpetrator’s or victim’s private residence?
Members of Congress consistently face calls to combat mass shootings. Without a clear method of defining the phenomenon, this can lead to different understandings of scope, imprecise and ineffective responses, and faulty measures of effectiveness. If mass shootings are defined too narrowly, it may be difficult to identify the shared contributing characteristics of the incidents that could be addressed by policy changes. If mass shootings are defined too broadly, any policy considerations may have to be broad enough to address a wide array of contributing characteristics associated with these incidents. Congress may debate the tradeoffs of adopting a definition of mass shootings narrowly, broadly, or at all. If policymakers choose to define what constitutes a mass shooting, they may also consider whether to adopt a single definition or multiple definitions that may cover specific types of shootings.
November 19, 2024
Jordan B. Cohen Analyst in Firearms Policy
How to Define Mass Shootings: Potential Policy Implications
Congressional Research Service
Three Debates About Defining Mass Shootings.............................................................................. 2
Fatalities and Mass Shootings ................................................................................................... 2
Other Felony Mass Shootings ................................................................................................... 5 Domestic Violence, Familicide, and Mass Shootings ............................................................... 5 Twelve Definitions of Mass Shootings ..................................................................................... 6
Mass Shootings by Definition Used, 2014 to 2023 ......................................................................... 8 Policy Considerations .................................................................................................................... 10
Table 1. Mass Shootings by Definition, 2014-2023 ........................................................................ 9
Author Information ........................................................................................................................ 13
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embers of Congress consistently face calls to combat mass shootings; however, there is no single, widely accepted definition of a mass shooting, and instead, there is a group of numerous, disparate, and uncoordinated definitions. Consequently, policy responses to
these incidents might be based on an unagreed upon and imprecise understanding of the issue. Accordingly, these policy responses may also lack the precision necessary to be successful.
Various entities have offered definitions related to mass shootings. For instance, the Federal Bureau of Investigation (FBI) defines “active shooter incidents,” and since 2000 has reported annually on active shooter incidents that meet “the federal definition of ‘mass killing’.”1 Active shooter incidents and mass killings are the closest concepts to mass shootings offered by federal law enforcement and require consideration of the status of the incident when it becomes known to law enforcement, the context in which the shooting occurs, and the number of fatalities.2 Active shooter incidents are those where “individuals [are] actively engaged in killing or attempting to kill people in a confined and populated area” involving the use of a firearm where the incident is ongoing and thus has “the potential for a response to affect the outcome.”3 Active shooter incidents that meet the federal standard for mass killings have at least three fatalities in a single incident, not including the shooter. If the police are notified of the incident after it has concluded or the event occurs in a sparsely populated public place, then it is not considered an active shooter incident.4 Incidents resulting from self-defense, gang violence, drug violence, domestic or residential disputes, hostage situations, or crossfire as a byproduct of another ongoing criminal act, as well as acts that do not put other people in peril, are also not considered active shooter incidents.5 In its most recent report on active shooter incidents, the FBI identified a total of 40 active shooter incidents that resulted in a mass killing between 2021 and 2023.6
Following the enactment of the Public Safety Officer Support Act of 2022 (P.L. 117-172), “mass shooting” is defined in 34 U.S.C. §10281(o)(1)(C) as a “multiple homicide incident in which not fewer than 3 victims are killed (i) with a firearm; (ii) during one event; and (iii) in one or more locations in close proximity.” This definition is intended specifically for cases in which an officer is claiming post-traumatic stress disorder (PTSD) or a similar trauma-induced condition as a line of duty injury for the purposes of qualifying for Public Safety Officer Benefits (PSOB) program benefits as a law enforcement officer, firefighter, or chaplain. It has not been applied for any other criminal justice purpose or benefits program for mass shootings outside of the PSOB program.7
In contrast to the definitions offered by the FBI and in 34 U.S.C §10281(o)(1)(C), nongovernment organizations have defined mass shootings in various ways. For example, the nonprofit organization Gun Violence Archive (GVA), which attempts to track every reported incident of gun violence in the United States using publicly available sources, defines a “mass shooting” as an incident where there is “a minimum of four victims shot, not including any shooter.” (The definition is agnostic to the number of fatalities.) Using this definition, the GVA
1 U.S. Department of Justice (DOJ), FBI, Active Shooter Incidents in the United States in 2023, Washington, DC, 2024, p. 1.
2 Ibid
3 Ibid
4 Pete J. Blair and Katherine W. Schweit, A Study of Active Shooter Incidents in the United States Between 2000 and 2013, Texas State University and the FBI, Washington, DC, 2014, pp. 4-5.
5 Ibid
6 DOJ, FBI, Active Shooter Incidents in the United States in 2023, pp. 2-3.
7 For more information on the Public Safety Officer Benefits Program, see CRS Report R45327, Public Safety Officers’ Benefits (PSOB) and Public Safety Officers’ Educational Assistance (PSOEA) Programs, by Scott D. Szymendera.
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counts 1,965 separate mass shootings during the same period examined by the most recent FBI active shooter incidents report (2023).8
Three questions illustrate some differences among the various definitions of a mass shooting:
1. Should there be a minimum number of fatalities or casualties for an event to be
defined as a mass shooting, and if so, what number should be used?
2. Is the shooting itself an end goal for the perpetrator, or does it occur within the
context of a broader crime?
3. Should domestic violence and familicide be considered mass shootings,
especially if they occur in a perpetrator’s or victim’s private residence?
The first debate in defining mass shootings is whether the occurrence or number of fatalities or casualties matters. In the 1980s, the FBI established a system to classify “multiple murder,” “mass murder,” “spree murder,” and “serial murder” incidents.9 The FBI defined “mass murder” as an incident where “four or more victims” were killed in one event and in one location. In 2008, it added detail to this definition, noting that mass murder is “described as a number of murders (four or more) occurring during the same incident, with no distinctive time period between the murders. These events typically involved a single location, where the killer murdered a number of victims in an ongoing incident.”10
In 2013, following the school shooting in Newtown, CT, Congress formalized a definition of “mass killing” in the Investigative Assistance for Violent Crimes Act of 2012 (P.L. 112-265) as three or more killings in a single incident. The act also authorized the Attorney General and FBI Director, at the request of a state or local law enforcement official, to assist in the investigation of violent acts, including mass killings and attempted mass killings in schools, malls, or other public places and nonfederal office buildings.11
In contrast, organizations like the GVA do not include a fatality requirement in their definition of “mass shootings” and instead define them by an injury threshold of four or more victims. The GVA notes that it “uses a purely statistical threshold to define mass shooting based ONLY [emphasis in the original] on the numeric value of 4 or more shot or killed, not including the
8 GVA, “Mass Shootings in 2021,” https://www.gunviolencearchive.org/reports/mass-shooting?year=2021; GVA, “Mass Shootings in 2022,” https://www.gunviolencearchive.org/reports/mass-shooting?year=2022; and GVA, “Mass Shootings in 2023,” https://www.gunviolencearchive.org/reports/mass-shooting?year=2023.
9 James Alan Fox and Jack Levin, Extreme Killing: Understanding Serial and Mass Murder, 3rd ed., Sage Publications, 2014, p. 23. Per the FBI, a “spree murder” is two or more murder victims slain, in one event, in two or more locations, without the offender “cooling-off” emotionally between murders. The event, however, can be of short or long duration. A “serial murder” is three or more separate homicidal events, with the offender cooling-off emotionally between the events. See DOJ, FBI, National Center for the Analysis of Violent Crime, Behavioral Analysis Unit, Serial Murder: Multi-Disciplinary Perspectives for Investigators, July 2008, p. 8, http://www.fbi.gov/stats-services/publications/serial- murder/serial-murder-july-2008-pdf.
10 DOJ, FBI, National Center for the Analysis of Violent Crime, Behavioral Analysis Unit, Serial Murder: Multi- Disciplinary Perspectives for Investigators, July 2008, p. 17, http://www.fbi.gov/stats-services/publications/serial- murder/serial-murder-july-2008-pdf.
11 See 28 U.S.C. §530C(b)(1). Note that none of the FBI definitions of “multiple murder,” “mass murder,” “spree murder,” and “serial murder,” nor Congress’s definition of “mass killing,” specified that a firearm needed to be used by the perpetrator.
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shooter. GVA does not parse the definition to remove any subcategory of shooting. To that end we don’t exclude, set apart, caveat, or differentiate victims based upon the circumstances in which they were shot.”12
Alternatively, scholars that study “foiled mass shootings” and “failed mass shootings” argue that neither injuries nor fatalities are necessary to understand the act of a mass shooting, but instead, are necessary to understand the outcomes of mass shootings.13 Scholars that study unsuccessful mass shootings define a “mass shooting” as,
a gun violence incident (or intended incident), perpetrated by (or intended to be perpetrated by) one or two offenders, targeting one or more public or populated locations, within a 24- hour period. At least some of the victims (or intended victims) must be chosen at random or for their symbolic value. If the perpetrator(s) does not kill four or more victims, they must demonstrate behavioral evidence suggesting mass victim intent. A public mass shooting, according to this perspective, excludes any incident involving familicide or other criminal activity.14
Proponents of a multiple-fatality threshold make two arguments as to why it should be part of a definition of a mass shooting. First, injury and death are qualitatively distinct. As a result, conflating them is misleading and elevates the level of fear when hearing individuals discuss incidents of firearm violence that do not result in multiple fatalities as mass shootings. Second, lowering the victim fatality threshold increases the risk of missing data when studying these incidents because there would be a large number of firearm violence incidents that need to be accounted for, especially when doing historical research on mass shootings.15
Notwithstanding these arguments, several issues exist when using fatalities as an element of the definition of mass shootings, including the following:
• There is no widely agreed upon number of fatalities to use if a definition has a fatality threshold. For instance, many organizations use four fatalities as the metric—including the Washington Post, the Associated Press, USA Today, and Everytown for Gun Safety.16 Alternatively, Mother Jones used the four-fatality threshold until 2013, at which point it switched to a three-fatality threshold to match the definition of “mass killing” provided by Congress. In addition, there are academic studies that require that a single person is killed and multiple
12 GVA, “General Methodology,” https://www.gunviolencearchive.org/methodology.
13 For an analysis of this debate, see Jason R. Silva, “Mass Shooting Outcomes: A Comparison of Completed, Attempted, Failed, and Foiled Incidents in America,” Deviant Behavior, vol. 43, no. 12 (2022), pp. 1487-1506.
14 Emily Ann Greene-Colozzi and Jason R. Silva, “Mass Outcome or Mass Intent? A Proposal for an Intent-Focused, No-Minimum Casualty Count Definition of Public Mass Shooting,” Journal of Mass Violence Research, vol. 1, no. 2 (September 2022), p. 28.
15 James Alan Fox and Emma E. Fridel, “Keeping with Tradition: Preference for the Longstanding Definition of Mass Shooting,” Journal of Mass Violence Research, vol. 1, no. 2 (September 2022). For more about why a fatality threshold can lead to more focused policy responses, see Jillian J. Turanovic et al., “A Comprehensive Assessment of Deadly Mass Shootings, 1980-2018,” National Institute of Justice: Investigator-Initiated Research and Evaluation on Firearm Violence, July 2022, p. 8; and Grant Duwe et al., “Forecasting the Severity of Mass Public Shootings in the United States,” Journal of Quantitative Criminology, vol. 38 (2022), pp. 385-423.
16 “There Have Been 22 Mass Killings with Guns in 2024,” Washington Post, September 4, 2024; Karina Zaiets and George Petras, “Mass killing database: Revealing trends, details and anguish of every US event since 2006,” USA Today, in partnership with the Associated Press and Northeastern University, August 18, 2022; and Everytown for Gun Safety, Mass Shootings in the United States, March 2023, https://everytownresearch.org/mass-shootings-in-america/.
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people are shot.17 No matter which one is chosen, there appears to be no research-based reasoning to prefer a certain fatality threshold.
• Because the impact of a mass shooting extends beyond the number of fatalities, using fatalities as a necessary condition to define mass shootings does not capture all of the injuries that first responders must react to and for which they have to prepare.18 One study of public shootings (defined as 10 or more people injured) found that among the victims of the shootings, 62.5% had injuries directly caused by the firearm, 40.7% underwent emergency department procedures, 36.5% were admitted to hospitals, 23.5% underwent surgery, and 10.4% underwent multiple surgeries.19 An approach that takes more public health issues into consideration when addressing mass shootings would generally consider the broader effects of the shootings.20 Using a minimum number of injuries as a threshold instead of fatalities may allow policymakers to evaluate more broadly the threat to public health stemming from these incidents.
• The relatively small sample size of mass shootings identified using a multiple- fatality threshold makes statistical evaluations of the various practice-based and policy responses challenging. Those seeking evidence-based solutions to mass shootings where multiple fatalities have occurred will find that the evidence base is limited due to research limitations associated with studying a relatively rare event. In the absence of an established research base upon which to build interventions, some have recommended that intervention development should be “a dynamic iterative process, involving stakeholders, reviewing published research evidence, drawing on existing theories, articulating programme theory, undertaking primary data collection, understanding context, paying attention to future implementation in the real world and designing and refining an intervention using iterative cycles of development with stakeholder input throughout.”21
• Not all incidents of firearm violence where shots are fired at multiple persons result in fatalities. This raises the question of whether a definition of mass shooting should consider the shooter’s intent. A definition that considers the perceived intent of the shooter and not just the fatality and injury outcomes would include incidents when an individual intends to shoot people indiscriminately and in a public place, carries out the shooting, but does not kill
17 Sarah Watson, “Mass Shootings, Fatality Thresholds, and Defining by Numbers: Political and Social Consequences,” Criminology and Criminal Justice, July 19, 2022, pp. 1-19.
18 Michael D. Kelleher, Flash Point: The American Mass Murderer (Westport, CT: Praeger, 1997), p. 2.
19 Matthew P. Czaja, Chadd K. Kraus, and Su Phyo, “Injury Characteristics, Outcomes, and Health Care Services Use Associated With Nonfatal Injuries Sustained in Mass Shootings in the US, 2012-2019,” Journal of the American Medical Association Network Open, vol. 5, no. 5 (2022).
20 Matthew P. Czaja et al., “Injury Characteristics, Outcomes, and Health Care Services Use Associated,” Journal of the American Medical Association Network Open, vol. 5, no. 5 (2022), pp. 1-13.
21 Alicia O'Cathain et al., “Guidance on how to develop complex interventions to improve health and healthcare,” BMJ Open, vol. 9, no. 8 (2019), p. 1. For firearm-specific analysis of this problem, see Louis Klarevas, Andrew Conner, and David Hemenway, “The Effect of Large-Capacity Magazine Bans on High-Fatality Mass Shootings, 1990-2017,” American Journal of Public Health, vol. 109, no. 12 (2019), pp. 1754-1761; and Daniel W. Webster et al., “Evidence Concerning the Regulation of Firearms Design, Sale, and Carrying on Fatal Mass Shootings in the United States,” Criminology and Public Policy, vol. 19, no. 1, p. 2020.
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anyone.22 For example, in a mall shooting in Davenport, IA, in July 2024, a 16- year-old allegedly discharged his firearm multiple times into a group of people; two people were injured, one of which was the shooter, and none were killed.23
A second debate affecting definitions of mass shootings is whether the shooting itself is an end goal for the perpetrator, or did it occur within the context of a broader crime? Other felony mass shootings are shooting incidents that occur during some other underlying criminal activity (e.g., armed robbery, gang violence, drug violence, terrorism, political violence). In other felony mass shootings, the perpetrator’s actions are related to other criminal activity with a larger financial, socioeconomic, or sociopolitical goal.
Deciding whether or not these types of incidents should be included in a definition of mass shooting depends, in part, on the types of mass shootings of interest to the public, public safety professionals, and policymakers. Other felony mass shootings can end up involving bystanders, but generally are defined as being geared toward the attack of specific, premeditated targets whose murders serve an overarching goal. By including these incidents as mass shootings, policies geared toward fighting gang violence, drug violence, and terrorism may be implicated when considering how to respond to other felony mass shootings, but they may not address the public and indiscriminate nature of some other types of mass shootings.
For its definition of “mass shootings,” Mother Jones excludes “shootings stemming from more conventionally motivated crimes” because it wants to take an “in-depth look at a distinct phenomenon” rather than “the broader problem of gun violence.”24 Conversely, the GVA includes other felony shootings because it wants to give “equal importance” to those who are injured and killed in any type of mass shooting incidents.25
A third debate is whether multiple-victim firearms-related domestic violence and multiple-victim familicide should be considered mass shootings. Mass shootings involving an offender who kills multiple family members—often, but not exclusively, in private residences—as well as mass shootings stemming from incidents of fatal domestic violence are considered familicide mass shootings.26
Some observers argue that the primary reason to include familicide mass shootings in overall mass shooting counts is that they make up a large percentage of fatal multi-victim shooting incidents. Domestic violence and familicide mass shootings have a higher fatality rate than other types of mass shootings.27 Discounting multiple-victim firearm-related domestic violence and
22 Emily Ann Greene-Colozzi and Jason R. Silva, “Mass Outcome or Mass Intent? A Proposal for an Intent-Focused, No-Minimum Casualty Count Definition of Public Mass Shooting,” Journal of Mass Violence Research, vol. 1, no. 2 (September 2022), pp. 27-41.
23 “Two Injured in Shooting at NorthPark Mall in Davenport Monday,” Quad City Times, July 22, 2024.
24 Mark Follman, Gavin Aronsen, and Deanna Pan, “A Guide To Mass Shootings in America,” Mother Jones, September 5, 2024.
25 GVA, “General Methodology,” https://www.gunviolencearchive.org/methodology.
26 Claire M. Renzetti and Jeffrey L. Edleson, “Familicide,” in Encyclopedia of Interpersonal Violence, vol. 2 (Thousand Oaks, CA: Sage Publications, 2008), p. 238.
27 Lisa B. Geller, Marisa Booty, and Cassandra K. Crifasi, “The Role of Domestic Violence in Fatal Mass Shootings in (continued...)
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multiple victim familicide ignores a substantively large portion of fatal mass shootings in the United States. For example, 41.85% of incidents where four or more people were killed by a firearm from 2014 to 2023 were familicide or domestic violence-related incidents (Table 1). Nonetheless, including familicides and domestic violence-related shootings in mass shooting statistics is not uniformly accepted because these types of shootings rarely involve the indiscriminate targeting of civilians in public places.28 For example, in its 2014 report analyzing active shooter incidents in the United States from 2000 to 2013, the FBI identified nine incidents where the perpetrator killed their family before moving to a more public location where the shooting continued.29
The intrinsic motivation and public impact are the main differences between familicide mass shootings and other mass shootings. Unlike many other types of mass shootings, familicides and domestic-violence related mass shootings are not necessarily premeditated nor do they typically include victims outside of the partner or family that is being targeted.30
A definition that includes the killing of family members comes from work published by USA Today in partnership with the Associated Press and Northeastern University. They define a “mass killing” as “the intentional killing of four or more victims—excluding the deaths of unborn children and the offender(s)—by any means within a 24-hour period. This definition includes cases involving all types (public, felony-related, and familicides).”31
CRS identified 12 different ways that mass shootings have been defined in the research literature and the media, which are described below. The first four definitions are fatality-based, definitions 5 through 8 are injury-based, and definitions 9 through 12 do not require a minimum number of fatalities or injuries.32
1. Fatal Mass Shootings. These are incidents of firearm violence where four or
more victims are killed. There are no other criteria (e.g., contexts, locations, shooter’s intent) that need to be met for this definition other than the shooter cannot be counted as one of the four victims killed.33
2. Fatal, Public Mass Shootings. These are incidents of firearm violence that occur
in relatively public places where four or more victims are killed—not including the shooter—and the victims are selected somewhat indiscriminately. Public places include workplaces, schools, restaurants, malls, and similar settings. The
the United States, 2014-2019,” Injury Epidemiology, vol. 8, no. 38 (2021), pp. 1-8; and James Alan Fox, “Trends in U.S. Mass Shootings: Facts, Fears, and Fatalities,” Journal of Contemporary Criminal Justice, vol. 40, no. 1 (2024), pp. 72-74.
28 William L. Sandel and M. Hunter Martaindale, “What Are We Talking About? Definitional Confusion Within Active and Mass Shooting Research,” Journal of Mass Violence Research, vol. 1, no. 2 (September 2022), p. 9.
29 J. Pete Blair and Katherine W. Schweit, A Study of Active Shooter Incidents in the United States Between 2000 and 2013, Texas State University and the FBI, Washington, DC, 2014, p. 19.
30 There are other shootings where some of the victims are known (e.g., school shootings and workplace shootings), but no data source, academic article, or policy paper identified by CRS discounted these events as mass shootings.
31 Karina Zaiets and George Petras, “Mass killing database: Revealing trends, details and anguish of every US event since 2006,” USA Today, in partnership with the Associated Press and Northeastern University, August 18, 2022.
32 These definitions are based in the policy research literature and media and are not legal definitions.
33 An example of this sort of definition can be found in Mark Follman, Gavin Aronsen, and Deanna Pan, “US Mass Shootings, 1982-2024: Data From Mother Jones’ Investigation,” Mother Jones, September 4, 2024, https://www.motherjones.com/politics/2012/12/mass-shootings-mother-jones-full-data/.
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violence itself is the end goal and is not a means to an end such as robbery, gang violence, drug violence, or terrorism, nor is it related to familicide or domestic violence. These types of incidents of firearm violence are excluded from this definition.34
3. Fatal, Public and Familicide Mass Shootings. These are incidents of firearm
violence that occur in public or private places; they include domestic violence- related shootings and familicide, and require that four or more victims are killed—not including the shooter. The violence is generally the end goal and not a means to an end such as robbery, gang violence, drug violence, or terrorism. These types of incidents of firearm violence are excluded from this definition.35
4. Fatal, Public and Other Felony Mass Shootings. These are incidents of firearm
violence that occur in public or private places and in which four or more victims are killed—not including the shooter. The violence can be a means to an end such as robbery, gang violence, drug violence, or terrorism, but it is generally not related to familicide or domestic violence, which are otherwise excluded from this definition.36
5. Public Mass Shootings. These are incidents of firearm violence that occur in
relatively public places, four or more victims are injured—not including the shooter—and the victims are selected somewhat indiscriminately. Public places include workplaces, schools, restaurants, malls, and similar settings. The violence is the end goal and is not a means to an end such as robbery, gang violence, drug violence, or terrorism, nor is it related to familicide or domestic violence. These types of incidents of firearm violence are excluded from this definition.37
6. Public and Familicide Mass Shootings. These are incidents of firearm violence
that occur in public or private places, they include domestic violence-related incidents and familicide that result in four or more victims being injured—not including the shooter. The violence is generally not a means to an end such as robbery, gang violence, drug violence, or terrorism. These types of incidents of firearm violence are excluded from this definition.38
7. Public and Other Felony Mass Shootings. These are incidents of firearm
violence that occur in public or private places and four or more victims are injured—not including the shooter. The violence can be a means to an end such as robbery, gang violence, drug violence, or terrorism, but it is generally not related to familicide or domestic violence, which are otherwise excluded from this definition.39
34 For example, see CRS Report R44126, Mass Murder with Firearms: Incidents and Victims, 1999-2013, by William J. Krouse.
35 For a study that differentiates between fatal familicide shootings, fatal public shootings, and fatal familicide shootings, see Emma E. Fridel, “A Multivariate Comparison of Family, Felony, and Public Mass Murders in the United States,” Journal of Interpersonal Violence, vol. 36, no. 3-4 (2017).
36 Ibid.
37 For an example of a quantitative study that investigates this definition, see Xue Lei and Cameron MacKenzie, “Quantifying the Risk of Mass Shootings at Specific Locations,” Risk Analysis, vol. 44, no. 4 (April 2024), pp. 868- 882.
38 For example, see Liza H. Gold, MD, “Domestic Violence, Firearms, and Mass Shootings,” The Journal of the American Academy of Psychiatry and the Law, vol. 48, no. 1 (2020), pp. 35-40.
39 The American School Shooting Study requires that at least one person be injured by a firearm, but it includes gang violence and other felonies. See Joshua D. Freilich et al., “Understanding the Causes of School Violence Using Open (continued...)
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8. Multiple Victim Shootings. These are incidents of firearm violence where four
or more victims are either killed or injured. There are no limitations on what conditions, locations, or reasons need to exist other than the shooter cannot be counted as one of the victims.40
9. Intended Mass Shootings. This is the broadest definition of a mass shooting.
Intended mass shootings are when a shooter targets multiple individuals with a firearm, regardless of whether they succeed in shooting anyone.41
10. Intended Public Mass Shootings. These are incidents of firearm violence that
occur in relatively public places and multiple people are somewhat indiscriminately targeted (not including the shooter). Public places include workplaces, schools, restaurants, malls, and similar settings. The violence itself is the end goal and is not a means to an end such as with robbery, gang violence, drug violence, or terrorism, nor is it related to familicide or domestic violence. These types of incidents of firearm violence are excluded from this definition.42
11. Intended Public and Familicide Mass Shootings. These are incidents of
firearm violence that can occur in public or private places, they include domestic violence and familicide incidents, and multiple people are targeted (not including the shooter). The violence itself is the end goal and is generally not a means to an end such as robbery, gang violence, drug violence, or terrorism. These types of incidents of firearm violence are excluded from this definition.43
12. Intended Public and Other Felony Mass Shootings. These are incidents of
firearm violence that can occur in public or private places and multiple people are targeted (not including the shooter). The violence can be a means to an end such as robbery, gang violence, drug violence, or terrorism, but it is generally not related to familicide or domestic violence, which are otherwise excluded from this definition.44
For this report, CRS used GVA data to assess how the definitions of mass shootings might impact policymakers’ understanding of the scope of mass firearm violence. The GVA provides the largest public-facing database of gun violence in the United States, and each incident of gun violence in
Source Data,” National Institute of Justice: National Criminal Justice Reference Service, Document Number: 301665, August 2021.
40 See GVA, “Mass Shootings in 2021,” https://www.gunviolencearchive.org/reports/mass-shooting?year=2021; GVA, “Mass Shootings in 2022,” https://www.gunviolencearchive.org/reports/mass-shooting?year=2022; and GVA, “Mass Shootings in 2023,” https://www.gunviolencearchive.org/reports/mass-shooting?year=2023.
41 Texas A&M’s Active Attack Data similarly defines an “active attack” as when an “individual or individuals is actively killing or attempting to kill multiple unrelated people in a public space.” See Texas A&M, “Active Attack Data,” https://www.activeattackdata.com/.
42 Emily Ann Greene-Colozzi and Jason R. Silva, “Mass Outcome or Mass Intent? A Proposal for an Intent-Focused, No-Minimum Casualty Count Definition of Public Mass Shooting,” Journal of Mass Violence Research, vol. 1, no. 2 (September 2022), p. 28.
43 In the context of rampage shootings, some scholars argue that casualty counts should be excluded and familicide included into definitions of mass violence. See Janelle Hawes and Eric Madfis, “Defining Rampage Violence Across Completion Status: Towards a More,” Journal of Mass Violence Research, vol. 1, no. 2 (September 2022), pp. 42-52.
44 For an example of this definition, see David Riedman and Desmond S. O'Neill, “K-12 School Shooting Database: Research Methodology,” Naval Postgraduate School, Center for Homeland Defense and Security, 2018.
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the database has been vetted by a team of professional researchers.45 The GVA has collected information from over 7,500 law enforcement, media, government, and commercial sources to provide near-real-time data about incidents of gun violence.46 Each of the incidents in its dataset includes, if available, 106 incident characteristics, and the data can be filtered allowing users to search for incidents by certain characteristics, such as the location of a shooting, the number of participants (both perpetrators and victims), and any relationship between the perpetrators and victims, among others. The GVA does not crowdsource or use internet bots to collect this data.47 CRS analyzed the gun violence data provided to it by the GVA, using the 12 definitions of mass shootings identified above.
There have been criticisms of the GVA; most relate to the organization defining “mass shooting” as “four or more people shot and killed in a single event, at the same general time and location not including the shooter.” The criticisms of GVA’s definition are that it is too broad and not representative of the more narrow world of mass shooting incidents.48 However, these criticisms are less concerning for the purposes of this report because CRS is considering multiple definitions of mass shooting and related concepts and is using GVA’s firearm violence data to assess how different definitions can affect descriptions of the frequency of mass firearm violence.
Table 1. Mass Shootings by Definition, 2014-2023
Type of Mass Shooting Number of Mass Shootings, 2014-2023
Fatality Threshold
Fatal Mass Shootings 258
Fatal, Public Mass Shootings 99
Fatal, Public and Familicide Mass Shootings 207
Fatal, Public and Other Felony Mass Shootings 143
Injury Threshold
Public Mass Shootings 3,769
Public and Familicide Mass Shootings 4,026
Public and Other Felony Mass Shootings 4,307
Multiple Victim Shootings 4,585
No Fatality or Injury Threshold
Intended Mass Shootings 10,580
Intended Public Mass Shootings 7,477
Intended Public and Familicide Mass Shootings 7,773
Intended Public and Other Felony Mass Shootings 10,259
45 The GVA uses a team of 20 researchers that vet and cross check reported incidents of gun violence with municipalities and “End of Year” reports of gun violence from large municipalities. They also measure and recheck their data against CDC and FBI data when their reports are released.
46 In contrast, the FBI’s UCR data is released several months after the conclusion of the year for which it collected the data.
47 For more information, see GVA, “General Methodology,” https://www.gunviolencearchive.org/methodology.
48 For example, see John R. Lott Jr., “The Truth About Mass Shootings,” RealClearPolicy, January 13, 2024, https://www.realclearpolicy.com/articles/2024/01/13/the_truth_about_mass_shootings_1004759.html.
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Source: Data received from the Gun Violence Archive on June 29, 2024; CRS verification of data completed on August 12, 2024.
Note: After reviewing and reverifying the GVA data, CRS only included the cases where it could find multiple sources from media, government, or local law enforcement confirming the information in the GVA archive. If CRS was able to confirm a shooting, but not the related incident characteristics, the shooting was included in the dataset but the flagged incident characteristic(s) was not.
A review of the GVA data reveals that counts of the total number of mass shootings vary greatly depending on the chosen definition (Table 1). According to CRS analysis of the GVA data, depending on the definition used, over the period from 2014 through 2023 there have been as many as 10,580 incidents of intended mass shootings with a firearm, or as few as 99 incidents of fatal, public mass shootings.
Fatal, public mass shootings (incidents of firearm violence that require a fatality and occur in a relatively public place, where the shooting is not a means to an end, nor is it related to a domestic dispute or familicide) make up 38.4% of all fatal mass shootings (where four or more people are killed). Public mass shootings comprise 82.2% of all multiple victim shootings (where four or more people in total are killed or injured). Intended public mass shootings make up 70.6% of intended mass shootings (where there is no threshold for fatalities or injuries).49
Conversely, definitions that include familicide and domestic violence related mass shootings that occur in public or private places made up 41.9% of all fatal mass shootings (where four or more people are killed) over the time period examined. Mass shootings related to familicide and domestic violence comprise 5.6% of multiple victim shootings (where four or more people in total are killed or injured). Intended mass shootings related to familicide and domestic violence comprise 2.8% of intended mass shootings (where there is no threshold for fatalities or injuries).50
In addition, mass shootings that occurred in public or private places and where the shooting was a means to some other end, such as armed robbery, terrorism, gang violence, or drug violence, made up 17.1% of fatal mass shootings, 11.7% of multiple victim shootings, and 26.3% of intended shootings (where there is no threshold for fatalities or injuries).51 These data suggest that efforts to reduce other firearm felonies could potentially address a non-trivial percentage of mass shootings and intended mass shootings.
It is challenging to consider how different policies might reduce the risk and severity of mass shootings without a clear understanding of what is meant by mass shootings. Even basic questions about trends in mass shootings are confounded by the variety of definitions used in the research literature and public discourse. For example, the RAND Corporation conducted an analysis of thousands of published studies evaluating state policies designed to reduce various types of gun violence. It found limited evidence that bans on high-capacity magazines and licensing and permitting requirements reduce mass shootings and fatalities.52 Primarily due to
49 2.78% of these fatal mass shootings combined familicide with armed robbery or drug violence.
50 0.047% of these multiple shootings combined familicide with armed robbery or drug violence.
51 0.025% of these intended shootings combined familicide with armed robbery, drug violence, or gang violence.
52 The RAND Corporation identified one study that showed limited evidence that bans on high-capacity magazines could reduce mass shootings. Nonetheless, when the study included year fixed effects to control for unmeasured trends in mass shootings, the association between high-capacity magazine bans and incidences and fatalities for mass shootings were no longer statistically significant. For the study, see Daniel W. Webster et al., “Evidence concerning the regulation of firearms design, sale, and carrying on fatal mass shootings in the United States,” Criminology & Public (continued...)
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methodological issues, the RAND Corporation also discovered no supporting evidence that the following policies have had any effect on reducing mass shootings: background checks, banning low-quality handguns, bans on the sale of assault weapons, firearm safety training requirements, firearms sales reporting requirements, lost or stolen firearm reporting requirements, waiting periods, child-access prevention laws, concealed-carry laws, gun-free zones, laws allowing armed staff in K-12 schools, and stand-your-ground laws.53 The lack of supporting evidence is largely because the use of different definitions in the reviewed studies resulted in contradictory and sometimes statistically insignificant estimates of how often mass shootings occur, how the rate of mass shootings changes over time, and underlying incident characteristics in mass shootings.54
To date, there have been 64 bills and resolutions introduced in the 118th Congress that have mentioned mass shootings and four of them have defined the phenomenon. All four of these had a three- or four-person victim requirement and none required fatalities.55 Given the strengths and weaknesses of each definition identified in this report, Congress could continue to consider if there is value in defining what constitutes a mass shooting. Specifically, Congress might consider whether a clear and widely agreed upon definition should be a part of any policy considerations aimed at reducing mass shootings across the United States, whether multiple definitions that capture unique but shared characteristics of mass shootings are needed to inform policy, or whether the existing environment in which multiple definitions are used is sufficient. If policymakers choose to specify a definition or a set of definitions, Congress could consider what characteristics of mass shootings are most important to include.
One consequence of defining mass shootings is the definition could potentially impact resources given to victims of these incidents and the local law enforcement agencies investigating them. For example, the Office for Victims of Crime (OVC) provides resources for victims of mass shootings.56 This includes grants to victims of terrorism and/or mass violence with funding from the Antiterrorism and Emergency Assistance Program.57 Defining mass shootings for this purpose could lead to an increase or decrease in the number of individuals eligible for these resources depending on the definition used. As another example, in P.L. 112-265, in which Congress
Policy, vol. 19, no.1, (February 2020), pp. 171-212. The RAND Corporation also identified two studies that provide limited evidence that permit-to-purchase requirements could reduce fatalities. Nonetheless, the RAND Corporation also notes that “incidents and fatalities were quite sparsely represented in the data,” that the “estimates and confidence intervals may be biased and unreliable,” and that one study contained “concerns related to having fewer than ten observations per parameter.” For the RAND Corporation’s discussion of the studies, see Samuel Peterson, “Effects of Licensing and Permitting Requirements on Mass Shootings,” RAND Corporation, July 16, 2024, https://www.rand.org/ research/gun-policy/analysis/license-to-own/mass-shootings.html. For the two studies referenced, see Daniel W. Webster et al., “Evidence concerning the regulation of firearms design, sale, and carrying on fatal mass shootings in the United States,” Criminology & Public Policy, vol. 19, no.1, (February 2020), pp. 171-212; and Michael Siegel et al., “The Relation Between State Gun Laws and the Incidence and Severity of Mass Public Shootings in the United States, 1976-2018,” Law and Human Behavior, vol. 44, no. 5 (October 2020), pp. 347-360.
53 Rosanna Smart et al., The Science of Gun Policy: A Critical Synthesis of Research Evidence on the Effects of Gun Policies in the United States, Fourth Edition, RAND Corporation, RRA243-9, Santa Monica, CA, July 16, 2024, https://www.rand.org/pubs/research_reports/RRA243-9.html.
54 Jaclyn Schildkraut and Adam Lankford, “Mass Shootings in the United States and Beyond,” in Routledge Handbook of Homicide Studies, ed. Kyle A. Burgason and Matt DeLisi (New York, NY: Routledge, 2024), pp. 140-165.
55 The search was conducted on August 8, 2024, and used Congress.gov’s “Legislation Text” search tool. The initial search looked for only the phrase “mass shootings.” To find the pieces of legislation that included definitions, the search terms used were “mass shootings means,” “mass shootings are defined as,” and “defines mass shootings.” The original 64 pieces of legislation were then examined individually to confirm the numbers.
56 DOJ, OVC, “View Resources for the Victims of Recent Mass Violence Incidents,” July 17, 2024, https://ovc.ojp.gov/news/announcements/view-resources-victims-recent-mass-violence-incidents.
57 For more information, see CRS Report R42672, The Crime Victims Fund (CVF): Federal Support for Victims of Crime, by Lisa N. Sacco.
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defined “mass killing” as an incident involving “three or more” fatalities, Congress provides certain law enforcement officials with assistance from the Attorney General, Secret Service, and U.S. Immigration and Customs Enforcement in investigating these mass killing incidents. Revising the definition of mass shootings for this purpose could increase or decrease the number local law enforcement agencies eligible for this support depending on the definition used. Furthermore, should Congress consider defining mass shootings, it may consider whether it should use the same definition that is used for public safety officers receiving benefits from PSOB. If Congress chooses a different definition, it could consider if that change should be applied to public safety officers receiving PSOB benefits.
If Congress does pursue establishment of a federal definition of mass shootings, it may also consider if it is worth appropriating funds to have a publicly accessible dataset focused on tracking mass shootings that begins to address the existing challenges of evaluating policy responses to mass shootings that rely on different definitions and different datasets. This could include consideration of whether a federal agency should track mass shooting data or if it should be done by a nongovernmental agency. The FBI has a long track record of collecting crime data from state, local, and tribal law enforcement agencies, but the process for collecting and reporting the data impacts the its timeliness. Alternatively, Congress could choose to authorize grants to researchers and have them track and make publicly available mass shooting data for research purposes.
How to Define Mass Shootings: Potential Policy Implications
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Jordan B. Cohen Analyst in Firearms Policy
Joseph Angert, CRS Research Assistant, helped verify the accuracy of the data in this report.
This document was prepared by the Congressional Research Service (CRS). CRS serves as nonpartisan shared staff to congressional committees and Members of Congress. It operates solely at the behest of and under the direction of Congress. Information in a CRS Report should not be relied upon for purposes other than public understanding of information that has been provided by CRS to Members of Congress in connection with CRS’s institutional role. CRS Reports, as a work of the United States Government, are not subject to copyright protection in the United States. Any CRS Report may be reproduced and distributed in its entirety without permission from CRS. However, as a CRS Report may include copyrighted images or material from a third party, you may need to obtain the permission of the copyright holder if you wish to copy or otherwise use copyrighted material.