The Federal Bureau of Investigation is tracking close to 150 Americans it believes traveled to Syria in recent months, potentially to join armed groups, FBI Director James Comey said on Tuesday.
He said Americans fighting with foreign jihadist groups are a top concern for the FBI because of the possibility they could return to the United States with the training, expertise and connections required to launch an attack on home soil.
"We have tracked coming up on close to 150 people who traveled from the United States to Syria, for all manner of motivations. A significant number of them to fight," he told reporters at a briefing in Boston.
"We are determined not to allow future lines to be drawn from a terrorist diaspora out of Syria to a future 9/11," he said.
The FBI was paying most attention to those Americans believed to have joined the militant group Islamic State, which has captured swaths of Syria and Iraq and has launched a sophisticated recruitment campaign on the internet, Comey said.
"There we see somewhere in excess of a dozen [Americans who have joined Islamic State] that we have a pretty good handle on," Comey said.
"I don't have high confidence that I see the entire universe," he added.
He declined to say how many Americans the FBI believes may already have returned to the United States after fighting for jihadist groups abroad.
"That is something that we track very closely," he said. "I don't want them to know too much about what we know."