According to a senior U.S. intelligence official, Abu-Salha is one of around 100 Americans who have traveled to Syria or have tried to do so as the civil war there grinds on. Not all of these Americans have joined al Qaeda — there are many factions fighting in Syria — but a number have done so.

Four other Americans have been publicly identified as having tried to join al Qaeda in Syria or have fought alongside the group, according to a count by the New America Foundation…

The problem in Syria is compounded by the fact that, according to both British counterterrorism officials and U.S. intelligence officials, senior al Qaeda members based in Pakistan have traveled to Syria to direct operations. According to a senior U.S. intelligence official, these include veteran members of al Qaeda “with strong resumes and full Rolodexes.”

As FBI Director James Comey explained in an interview with The Washington Post a month ago, officials are determined to prevent the kind of “blowback” from Syria that occurred previously from foreign fighters who had trained in Afghanistan in the years before 9/11.