Departed, Returned, Inspired

The 57 homegrown IS-plots can be broken into three rough categories based on the courses of action that plotters pursued as they attempted to support the terrorist group. The first two categories focus on foreign fighters, the last on people willing to do harm in the United States: 
The Departed — American 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 return 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. Beyond the Departed, Returned, and Inspired

Aside from the three categories based on the courses of action that IS supporters follow, at least two other sorts of IS foreign fighters pose some threat to U.S. interests. The Lost: unidentified Americans who fight in the ranks of the Islamic State. Such individuals may come home after fighting abroad and remain unknown to U.S. law enforcement. Some American IS fighters will never book a trip back to the United States. Finally, some American IS supporters will perish abroad. 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 from abroad. The following discussion is not intended as an exhaustive analysis of federal law enforcement counterterrorism efforts in the homeland. Preempting Terrorists

Preemption of terrorist activity by U.S. law enforcement can be broadly described in terms of screening and 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 .
Analysis of publicly available information on homegrown violent jihadist activity in the United States since September 11, 2001, suggests that the Islamic State (IS) and its acolytes may pose broad challenges to domestic law enforcement and homeland security efforts. Homegrown IS-inspired plots can be broken into three rough categories based on the goals of the individuals involved. The first two focus on foreign fighters, the last on people willing to do harm in the United States:
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.
At least two other categories of IS foreign fighters pose some threat to U.S. interests:
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. 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.
Federal law enforcement has numerous approaches to go after each of these categories of terrorist actors. These include the following:
Watchlisting—the federal counterterrorism watchlisting regimen effectively attempts to shrink "the lost" category described above. Preemption—efforts geared toward preemption of terrorist activity 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, also known as countering violent extremism.