Dallas - FBI Director James Comey says it's difficult for federal authorities to track how many Americans are trying to go to Syria to join the civil war there.
Comey spoke on Monday at an event in Dallas. He talked about so-called homegrown terrorists who use the internet to connect with others interested in "jihad".
He said FBI agents try to distinguish between people only chatting about potentially extremist ideologies and those taking active steps to engage in terrorism.
The FBI and other agencies have arrested several people across the United States this year on charges that they discussed trying to go abroad to join terrorist groups.
In separate Texas cases this year, a University of Texas student and a Central Texas father were accused of planning to engage in jihad.