Last year, officials were estimating “dozens” of American militants had tried to join the various militant groups in Syria, including ISIL.
Comey, who spoke Monday at the FBI's field office in Dallas, talked about so-called homegrown terrorists or "lone wolves," who use the Internet to connect with others interested in war.
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.
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.
Well over 1,000 ISIL militant are known to have come from EU member nations, with large numbers from France.
Previously, the big concern with such rebels was that when the Syrian war eventually ended, ISIL-linked militant would return to the West with EU and US passports that gave them easy access to targets most radicals don’t have.
The US began its airstrikes against the militants on Friday, taking out several of their checkpoints and armed trucks as well as an armed personnel carrier.
However, US Joint staff operations director Lieutenant General William Mayville said the airstrikes are unlikely to affect the militants’ “overall capabilities” or their operations elsewhere in Iraq and Syria.
Meanwhile, President Barack Obama reiterated that the US is pursuing "limited military objectives" in Iraq.
The US military announced that it has no plans to expand airstrikes in northern Iraq beyond protecting its facilities and citizens from ISIL.
"There are no plans to expand the current air campaign beyond the current self-defense activities," senior Pentagon official Lieutenant General William Mayville told reporters on Monday.
SHI/SHI