FBI Director James Comey on Monday warned that Americans who go to Syria to fight with extremists may become terrorists when they return home.
“What we see in Syria is the creation of a safe haven, a training ground, and potentially a launching ground for terrorists,” he said.
Comey spoke to reporters during a ceremony marking the 100-year anniversary of the bureau’s Dallas office.
The FBI is tracking more than 100 Americans who have gone to Syria as armed fighters, he said.
Comey first estimated he knew of about 100 people who had gone to and from Syria, or had been caught trying to so. But he later hedged on that figure.
"When I give you the number of more than 100, I can't tell you with high confidence that's a 100 of 200, that's a 100 of 500, that's a 100 of a 1,000 or more, because it's so hard to track," Comey said.
He said FBI agents tried to distinguish between people only chatting about potentially extremist ideologies -- whom he called "mouth-runners" -- and those taking active steps to engage in terrorism.
"This is a great country with lots of traditions of protecting mouth-running," he said. "We should continue that. But those who are inclined to cross the line, I've got to focus on them.
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.
The Central Texas man, Michael Todd Wolfe, was arrested in June at Houston's Bush Intercontinental Airport, where federal agents say he was waiting with his family to fly to Denmark on his way to the Middle East. Wolfe has since pleaded guilty to one count of providing material support or resources to terrorists. The University of Texas student, Rahatul Khan, has also pleaded guilty to a federal charge.
Syria shares a long border with Iraq, where the Islamic State of Iraq and Syria (ISIS) continues its march across Northern Iraq. American warplanes have bombed ISIS positions in recent days near the city of Erbil, where the United States has a consulate.
"The ‘going’ there matters tremendously but what concerns me is the 'coming,’” he said. “There will be a terrorist diaspora out of Syria at some point.”
He said Western European countries face the same threat.
“There are thousands of terrorists who have flowed from all over Western Europe, North America, throughout Asia, to Syria where they are getting the worst kind of training, making the worst kind of relationships,” Comey said.
Comey, who has served as FBI director for nearly one year, has visited many bureau offices around the country in his first months on the job. The Dallas visit on Monday was his 37th trip to an FBI field office.