Authorities were alarmed to learn that Moner Mohammad Abusalha, the first - and so far, only - American to carry out a suicide bomb attack in Syria, returned to his home in Florida between the time he first traveled to Syria to train with an al-Qaeda offshoot and the day he detonated explosives strapped to his body.
Abusalha didn't carry out an attack on the U.S. But he returned to U.S. soil as a fighter fully trained by the al-Qaeda affiliated Nusra Front, and under no suspicion. That's the prospect that is so 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 Juan Zarate, CBS News' National Security Analyst.
Multiple experts said the ease with which Abusalha re-entered the U.S. after training in Syria is cause for concern. As Tom Sanderson, a terrorism expert at the Center for Strategic and International Studies, told CBS News, it's very hard to tell which of the estimated 100 to 200 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.
It's not just American fighters, either. Europeans who have become radicalized pose an equally great threat because they can enter the U.S. without obtaining a visa if their radicalization went undetected and they were not placed on any watch list. Between 1,500 and 2,500 Sunni extremists from Europe have traveled to Syria since January 2012 to join groups like the Nusra Front and the Islamic State of Iraq and Syria (ISIS), the militant group that has taken over the northern part of Iraq, estimates Seth Jones, a former U.S. counter-terrorism official.
One reason it's so hard to track which westerners have gone to join the fight is because of a lack of robust intelligence collection in the region.
"The U.S. had a pretty good intelligence collection capability in Afghanistan in Iraq, including human intelligence collection, including people on the ground, including special operators," Jones, who is now with the Rand Corp. think tank, told CBS News. "I don't think it's a highly resourced effort right now."
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Zarate also said that it can be difficult for the intelligence community to determine which fighters might pose a threat if they display no signs of becoming radicalized.
"The FBI really, despite the lore out there, isn't an MI-5. They're not outfitted to follow each and every suspect for 24 hours, 7 days a week if they haven't committed a crime or aren't under investigation for having committed a crime," Zarate said. "If somebody's traveled abroad, potentially connected with a terrorist group, but we don't have evidence of it, we don't know that that's the case, the FBI is not going to track them and isn't going to know and the problem."
In addition to sending more intelligence resources to training camps in Syria, Jones said the U.S. could do more to work with communities in the U.S. to identify individuals who might be susceptible to recruitment by jihadist networks.
Syria is also easy to reach through Turkey and other points in Europe, adding a layer of difficulty in tracking Americans and Europeans who have traveled there. That was the route that Abusalha chose, Zarate noted, when he traveled to Syria with relatively little money.
There haven't been any successful attacks yet. But as experts note, even one radicalized American who has flown under the radar can be cause for concern.
Plus, the longer the U.S. remains actively involved in the Middle East - whether it is with American troops in Afghanistan, military advisers in Iraq or vocally in support of Israel in the Gaza conflict - the more it could become a target for terror groups who have been more focused on sectarian conflicts.
"Given Abusalha, given the massive increase in foreign fighters in the region , given the potential involvement of U.S. forces in Iraq...the chances of an attack on the U.S. are increasing because of those factors," Sanderson said.