FBI counterterrorism agents followed dozens of potential terrorists around the United States full time over the summer and disrupted activities pursued by many of them, FBI Director James Comey told a congressional committee on Thursday.
Comey also told the Senate Committee on Homeland Security and Governmental Affairs that U.S. investigators knew of many U.S.-based Islamic terror suspects who used encrypted communications.
Nick Rasmussen, director of the National Counterterrorism Center, also speaking to the panel, estimated that about 250 Americans had traveled to fight or train in parts of Iraq and Syria where the Islamic State have declared a caliphate.
But he noted this was relatively small, given the total of about 5,000 westerners who United States and allied governments believed have joined the fight in the last three years.
Rasmussen said the United States and its allies estimate that as many as 28,000 foreigners have gone to the region since 2012. A large majority of them came from countries in the region.
Security sources say that to follow someone without being detected, government agencies needed crews of up to 36 watchers for a 24-hour shift. This stretches resources in European countries, even in those with strong counter-terrorism agencies.