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U.S. can't track Westerners fighting in Syria, top intel official says

WASHINGTON -- A top American intelligence official is acknowledging that the U.S. has difficulty tracking the movements and activities of Westerners in Syria who have joined rebels fighting President Bashar Assad.

Matt Olsen, who directs the National Counter Terrorism Center, told a House committee Wednesday that intelligence agencies have very little idea where foreign fighters go and what they do once they reach Syria, so they can't estimate how many have joined the Islamic State of Iraq and Syria (ISIS) or other extremists.

Flash Points: Why are foreign fighters so hard to track?

The U.S. is able to track Westerners who travel to and from Syria, Homeland Security Secretary Jeh Johnson says. But inside Syria, there are intelligence gaps.

The U.S. estimates that about 15,000 foreign fighters have flown to Syria, up from a previous estimate of 12,000.

The U.S. government has positively identified a relatively small number of Americans - fewer than 12 - who have joined ISIS, but precise numbers are unavailable and intelligence assessments, while educated, are still estimates due to limited U.S. intelligence in Syria, CBS News correspondent Bob Orr reports.

Flash Points: Why are foreign fighters gathering in Iraq and Syria?

On Monday, Secretary of State John Kerry said that stopping the flow of foreign fighters joining ISIS will be far more important than airstrikes in the mission to stamp out the extremist group.

Tom Sanderson, a terrorism expert at the Center for Strategic and International Studies, told CBS News that it's very hard to tell which Americans who have traveled to Syria since 2011 might come home and pose a threat.

"To know that would be indicative of a very deep level of intelligence on the person, to be able to know that he, most likely, would have the inclination to attack the U.S. upon return or to attack a U.S. equity overseas," Sanderson said.

The prospect of U.S. extremists returning from fighting in Syria is worrisome for officials.

"Is this a local fight? Are they going there to really just battle the Syrian forces and to topple [Syrian President Bashar] Assad or is there even a small percentage of these folks who are going to return home and turn their sights on western victims and their fellow citizens? And that's the real concern for U.S. counterterrorism officials," said CBS News national security analyst Juan Zarate.

American carried out suicide bombing in Syria, U.S. officials say

Moner Mohammad Abusalha, an American who grew up a basketball fan in Vero Beach, Florida, killed 16 people and himself in a suicide bombing attack against Syrian government forces in May. U.S. officials say he was on their radar screen but acknowledge he traveled from Syria to the United States before the attack without detection. Had he attacked in the U.S. instead of Syria, it's unclear whether he would have been stopped.

Abusalha was recorded in a series of videos before his attack. In one of them, he addresses the U.S. public in American-accented English.

"You think you are safe? You are not safe," he said. "We are coming for you, mark my words."