The Islamic State’s Acolytes and the Challenges They Pose to U.S. Law Enforcement: In Brief

Summary

Analysis of U.S. counterterrorism investigations since September 11, 2001 (9/11), suggests that the Islamic State (IS) and its acolytes may present broad challenges for domestic law enforcement. These challenges involve understanding and responding to a variety of terrorist actors who can be placed into five categories:

The Departed—Americans, often described as foreign fighters, who plan to leave or have left the United States to fight for the Islamic State.

The Returned—American foreign fighters who trained with or fought in the ranks of the Islamic State and come back to the United States, where they can potentially plan and execute attacks at home.

The Inspired—Americans lured—in part—by IS propaganda to participate in terrorist plots within the United States.

The Others—Foreign IS adherents who radicalize in and originate from places outside of the United States or non-American foreign fighters active in the ranks of the Islamic State. These persons could try to enter the United States when done fighting abroad.

In this conceptualization, the departed, the returned, the inspired, and the others are known or suspected by law enforcement as terrorists. This suggests an additional category:

The Lost—Unknown Americans who fight in the ranks of the Islamic State but do not plot terrorist attacks against the United States. Such individuals may come home after fighting abroad and remain unknown to U.S. law enforcement. Additionally, some American IS fighters will never book a trip back to the United States. Finally, some American IS supporters will perish abroad.

Federal law enforcement has numerous approaches to go after each of these categories of terrorist actors. These include the following:

The federal counterterrorism watchlisting regimen. The regimen effectively attempts to shrink the “the lost” category described above.

Efforts geared toward preemption of terrorist activity. These can be broadly described in terms of interdiction (stopping a suspected terrorist from entering the United States, for example), law enforcement investigation, and government activities aimed at keeping radicalized individuals from morphing into terrorists.