The radicalization of Americans by Islamic State and other groups, particularly through sophisticated use of social media, is a top concern for the FBI as it grapples with evolving terrorism threats, Director James Comey said on Thursday.
Like other militant groups, Islamic State, also known as ISIL, has called for lone wolf attacks in Western countries and has specifically encouraged attacks on soldiers, law enforcement and the intelligence community, Comey said at an appropriations subcommittee budget hearing.
Comey cited the group's continuing efforts to recruit Americans to join Islamic State fighting in Syria and Iraq, then have them return to the United States to commit acts of terrorism.
"ISIL's widespread reach through the Internet and social media is most concerning as the group has proven dangerously competent at employing such tools for its nefarious strategy," Comey told the panel as he presented his $8.48 billion budget request for fiscal year 2016.
"This poses an enormous challenge to us: to find the people who are responding to that siren song," Comey said.
U.S. National Intelligence Director James Clapper said early this month that about 180 Americans have traveled to Syria to join Islamist militants and about 40 have returned to the United States.
Comey called the threats posed by such foreign fighters and from homegrown violent extremists "extremely dynamic."