The September 11, 2001 terrorist attacks on the United States gave new momentum to European Union (EU) initiatives to combat terrorism and other cross-border crimes such as drug trafficking, human trafficking, and financial fraud. For many years, EU efforts to address such challenges were hampered by national sovereignty concerns, insufficient resources, and a lack of trust among law enforcement agencies. However, the terrorist attacks and the subsequent revelation of Al Qaeda cells in Europe changed this status quo as it became increasingly evident that the EU's open borders and different legal systems allowed terrorists and other criminals to move around easily and evade arrest and prosecution. Thus, EU officials renewed their efforts to harmonize national laws and bring down traditional barriers among member states' police, intelligence, and judicial authorities. As part of this initiative, the EU has also sought to enhance ongoing cooperation with U.S. law enforcement and judicial authorities so that information can be meaningfully shared and suspects apprehended expeditiously. The March 11, 2004, terrorist bombings in Madrid, Spain, injected a greater sense of urgency into EU efforts to boost police and judicial cooperation within the EU and improve EU external border controls. Despite the EU's progress, however, the Union faces a number of political, legal, and cultural hurdles as it seeks to introduce more effective law enforcement tools. For example, some member states were slow to implement the EU-wide arrest warrant -- which eliminates extradition proceedings among member states for 32 offenses, including terrorism -- and other EU legislative instruments to counter terrorism. National police and intelligence services remain reluctant to share information. Contentious issues such as the use of the death penalty in the United States and different data protection regimes have also posed challenges at times to more robust U.S.-EU cooperation. The 9/11 Commission recommended that the United States "should engage other nations in developing a comprehensive coalition strategy against Islamist terrorism;" the House and Senate have passed intelligence reform legislation ( H.R. 10 and S. 2845 ) with elements that seek to enhance international cooperation against terrorism. The Bush Administration, backed by Members of Congress, supports EU efforts to strengthen its counterterrorism capabilities, and welcomes initiatives aimed at complementing and improving existing bilateral cooperation between U.S. and EU member states' intelligence and law enforcement agencies. The United States has concluded two information-sharing agreements with Europol, signed two treaties with the EU on extradition and mutual legal assistance, and has been working to improve cooperation with EU border control officials. Some critics question, however, whether U.S.-EU-wide cooperation will add much value to existing bilateral law enforcement relationships. This report will be updated as events warrant. Also see CRS Report RL31612(pdf) , European Counterterrorist Efforts: Political Will and Diverse Responses in the First Year After September 11 , coordinated by Paul Gallis.