The following is from a Sept. 4, 2013 Congressional Research Service report:
The September 11, 2001, terrorist attacks on the United States and the subsequent revelation of Al Qaeda cells in Europe gave new momentum to European Union (EU) initiatives to combat terrorism and improve police, judicial, and intelligence cooperation among its member states.
Other deadly incidents in Europe, such as the Madrid and London bombings in 2004 and 2005 respectively, injected further urgency into strengthening EU counterterrorism capabilities and reducing barriers among national law enforcement authorities so that information could be meaningfully shared and suspects apprehended expeditiously.
Among other steps, the EU has established a common definition of terrorism and a common list of terrorist groups, an EU arrest warrant, enhanced tools to stem terrorist financing, and new measures to strengthen external EU border controls and improve aviation security.
As part of its drive to bolster its counterterrorism capabilities, the EU has also made promoting law enforcement and intelligence cooperation with the United States a top priority. Washington has largely welcomed these efforts, recognizing that they may help root out terrorist cells both in Europe and elsewhere, and prevent future attacks against the United States or its interests abroad.
U.S.-EU cooperation against terrorism has led to a new dynamic in U.S.-EU relations by fostering dialogue on law enforcement and homeland security issues previously reserved for bilateral discussions.
Contacts between U.S. and EU officials on police, judicial, and border control policy matters have increased substantially since 2001. A number of new U.S.-EU agreements have also been reached; these include information-sharing arrangements between the United States and EU police and judicial bodies, two new U.S.-EU treaties on extradition and mutual legal assistance, and accords on container security and airline passenger data.
In addition, the United States and the EU have been working together to curb terrorist financing and to strengthen transport security.