The U.S. believes there are about a dozen Americans fighting alongside extremist groups in Syria, not more than 100, as has been cited for months.

That's not to say there is no concern about these other 88 or so Americans who officials say have been killed, arrested, traveled or attempted to travel to join the fight. But the U.S. only knows of about 12 who are currently in Syria fighting, FBI Director James Comey said Thursday.

Comey also said the U.S. believes it knows the identity of the British-accented masked man in the videos depicting the beheadings of two American journalists and a British aid worker. He declined to name the man or reveal his nationality.

He did not address whether the U.S. believes the man actually carried out the killings. The full beheadings are not shown in the videos, but the English-speaking militant holds a long knife and appears to begin cutting the three men, American reporters James Foley and Steven Sotloff and British aid worker David Haines.

The figure of 100 American militants joining the fight in Syria had taken on an urban legend status over the past few months as the Obama administration made its case to the American public for military action in Iraq and Syria. It's unclear what significance the discrepancy has as far as Americans' support for the U.S. military action, which so far has been strong.

"When I use a number of more than 100, that means people who have gone and come back, people who have attempted to go and we locked them up, people who have gone and stayed," Comey said during an interview with reporters at FBI headquarters. "The figure that I've been operating with is, ballparkish, a dozen still there fighting with terrorist groups."

But that breakdown has been absent from other Obama administration accounts.

"Thousands of foreign fighters have flocked to Syria over the past three years," Matthew Olsen, the director of the National Counterterrorism Center, said last week. "This includes more than 2,000 Europeans and more than a hundred Americans."

The head of the House Homeland Security Committee, Rep. Michael McCaul, R-Texas, described it this way: "Coupled with 30,000 of these ISIS fighters, 15,000 are foreign fighters, over 100 American U.S. citizens. Many of these fighters have Western passports." He was using one of the acronyms for the Islamic State group.

The senior Republican on the Senate Armed Services Committee, James Inhofe of Oklahoma, told Defense Secretary Chuck Hagel during a hearing, "It's estimated that at least 2,000 fighters hold Western passports and at least 100 are U.S. citizens." Hagel never corrected him or clarified the 100 figure.

In June, Comey also cited the 100 figure during interviews with reporters at the FBI's Minneapolis field office.

"The 100 or so people that we are tracking, who have traveled, gone and stayed, gone and come back, are from everywhere in the United States, from all walks of life, making it enormously challenging for us," Comey said.

But Comey's explanation was a mouthful, and it's been short-handed into what is now known to be an exaggerated number of Americans fighting in Syria.

While Olsen didn't specifically say that 100 Americans are currently fighting with extremists in Syria, it has been interpreted that way.

"This figure includes well over 1,000 European citizens, and more than 100 Americans," Rep. William Keating, D-Mass., said last week. Keating said he was citing figures provided by the National Counterterrorism Center.

National Counterterrorism Center spokesman Joe Vealencis said the "more than 100" figure refers to the total number of Americans who have tried to go to Syria to fight or have gone and come back.

U.S. intelligence officials estimate there are about 15,000 foreign fighters in Iraq and Syria fighting with extremist groups, including the Islamic State group. Officials long have been concerned that Americans are joining the fight. At least two Americans have been killed in the fighting, and at least 13 have been arrested.

The actual number is a "big unknown," said Derek Harvey, a former Defense Intelligence Agency official who advises the Pentagon's Central Command.

"The director is talking about information that they have that is solid," Harvey said of Comey's 12 Americans statistic. "But there's a lot of unknowns, and I think that's why there's a lot of concern."

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Associated Press writer Amy Forliti in Minneapolis and Stephen Braun in Washington contributed to this report.