For background information and citations, see U.S. Congress, House Select Committee on Assassinations, Report: Findings and Recommendations, H.Rept. 95-1828, Part 2, 95^th Cong., 2^nd sess. (Washington: GPO, 1979); U.S. Department of the Treasury, Background Information on the White House Security Review (Washington: Department of the Treasury, 1995), pp. 51-101; U.S. National Commission on the Causes and Prevention of Violence, Final Report (Washington: GPO, 1969), pp. 122-124; James E. Kirkham, et al., Assassination and Political Violence: A Report to the National Commission on the Causes and Prevention of Violence (Washington: GPO, 1969), p. 22; U.S. President's Commission on the Assassination of President John F. Kennedy, Report (Washington; GPO, 1964), pp. 504-515; James W. Clarke, American Assassins: The Darker Side of Politics (Princeton: Princeton University Press, 1982); [author name scrubbed], "Presidential Assassinations and Assaults: Characteristics and Impact on Protective Procedures," Presidential Studies Quarterly, vol. 11, Fall 1981, pp. 545-558, and "Origins of Secret Service Protection of the President: Personal, Interagency, and Institutional Conflict," Presidential Studies Quarterly, vol. 18, Winter 1988, pp. 101-128; Philip H. Melanson, The Secret Service (New York: Carroll and Graf, 2002); Facts on File Yearbook, 1994, p. 877, and 1995, p. 361; "Police Shoot Man Near White House," Washington Post, February 8, 2001, pp. A1, A10; extensive press coverage of the terrorist attacks on the Pentagon and World Trade Center, along with the possible targeting of the White House or Capitol, on September 11, 2001, including U.S. News and World Report, Special Report, September 14, 2001, pp. 17-27, and Newsweek, Extra Edition [September 12, 2001], p. 32; press coverage in 2005 and 2006 of an incident involving President George W. Bush in Tbilisi, Republic of Georgia; and congressional hearings on a security breach at a state dinner at the White House on November 24, 2009, U.S. House Committee on Homeland Security, The United States Secret Service and Presidential Protection: An Examination of a System Failure, hearings, 111^th Cong., 1^st sess, Dec, 2, 2009.

Illustrative of these are: apprehension about President Madison (and his wife) being taken hostage, when British troops invaded Washington, DC, in 1814; rocks thrown at President Tyler by an intoxicated painter; fears of kidnapping and assassination of Abraham Lincoln, beginning with his journey to Washington, DC, for the inauguration in 1861; shots fired at President Lincoln by Confederate troops, when he was observing a battle between them and Union forces stationed at Fort Stevens, July 11-12, 1864; allegations of a possible kidnapping of President Cleveland's children, along with unknown intruders at his summer vacation property, in 1894, while at the same time he was threatened by Colorado gamblers who had traveled to Washington, and separately, by sympathizers of Coxey's Army, who wanted to "remove" Cleveland from office; stalking of President Nixon, by the same person who later, in 1972, shot presidential candidate George Wallace; a planned airplane attack on the White House, which literally never got off the ground, because the pilot (and would-be assassin) was killed before the craft was airborne, in 1974; a car, driven by a man wired with explosives, crashing through the White House gates, also in 1974; discovery of a truck containing explosives outside a building that was being used as the "mobile White House," when President Clinton was visiting Belfast, Northern Ireland's capital; a shooting incident outside the White House fence on February 7, 2001, in which the assailant, who was wounded, was charged with assaulting a federal officer; on September 11, 2001, an aborted attempt to use a hijacked commercial airliner possibly to crash into the White House or the Capitol, as three other airplanes had been used to attack the Pentagon and the World Trade Center in New York; occasional security breaches of the White House grounds and state functions; along with hundreds of threats yearly against the President, family members, and high-ranking officials.

Protection of these individuals, along with others, falls to the U.S. Secret Service (18 U.S.C. 3056), which can call upon other agencies to assist it. This duty began in 1894 with President Cleveland and his family; other specific assignments have been added since then to include vice presidents, immediate family members, former presidents, and persons serving as representatives of the president abroad, among others. For background and further citations, see CRS Report RL34603, The U.S. Secret Service: An Examination and Analysis of Its Evolving Missions, by [author name scrubbed].

Speculation had long existed that President Zachary Taylor, who was a foe of extending slavery, was poisoned by pro-slavery conspirators in 1850, a suspicion arising in part because his symptoms at the time of his death resembled those of arsenic poisoning. A 1991 laboratory analysis of Taylor's remains, however, found only minuscule levels of arsenic, which could not have caused illness, let alone death. A forensic anthropologist concluded that President Taylor died of natural causes, perhaps from contaminated food he had eaten or possibly from the treatments—cathartics and laxatives—for gastroenteritis and acute diarrhea. William R. Maples and Michael Browning, Dead Men Do Tell Tales (New York: Doubleday, 1994).

Regarding the assassination of President John F. Kennedy, the 1964 Warren Commission inquiry concluded that Lee Harvey Oswald acted alone. U.S. President's Commission, Report, p. 22. A 1979 congressional investigation determined, however, that the President "was probably assassinated as a result of a conspiracy. The Committee is unable to identify the other gunman or the extent of the conspiracy." House Select Committee on Assassinations, Report, p. 1. In addition, a conspiracy was initially suspected in President McKinley's murder, in 1901, because his assailant was a self-described "anarchist," a designation applied to assassins of European leaders in the same time period. Extensive investigations by the U.S. Secret Service, as well as Buffalo and New York State law enforcement agencies and prosecutors, however, found no evidence that McKinley's assailant was aided by anyone else.