As Moner Mohammad Abu-Salha packed explosives into a truck last month to embark on a suicide attack, his family in Florida was apparently unaware that he had even wandered into Syria's civil war.
For months, he had been sending emails indicating he was in Jordan, possibly caring for the wounded but far from the fight. "I'm here now, doing fine," the emails would say, according to Taher Husainy, a family friend in Vero Beach. "I'm here for a good cause, doing good things."
American counterterrorism agencies were only slightly better informed. U.S. officials said they knew that Abu-Salha had crossed into Syria, but they had scant intelligence on his activities there or his association with an al-Qaida affiliate until he appeared last week in an online martyrdom video.
The inability to track Abu-Salha reflects what U.S. officials describe as a worrisome blind spot for intelligence agencies struggling to monitor a surging flow of foreign fighters into and often out of a conflict dominated by Islamist militants.
U.S. officials said dozens of fighters from the United States, and much larger numbers from Europe and the Middle East, all but disappear from view once they are inside Syria's borders. The officials described Syria as a daunting environment for espionage.
Amid estimates that as many as 12,000 foreigners have gone to Syria, the opaque nature of the conflict has complicated efforts to determine how many might have become dangerously radicalized or to account for them if and when they return home.
Some U.S. counterterrorism officials said the suicide bombing by Abu-Salha was evidence that the Nusra Front, the al-Qaida-linked group that asserted responsibility for the attack, remains focused on ousting Syrian President Bashar Assad rather than taking advantage of an American recruit to launch a plot against the West. The officials spoke to the Washington Post on condition of anonymity to discuss U.S. surveillance efforts.
"It's a game changer," said Martin Reardon, who worked on FBI counterterrorism assignments for a decade before retiring in 2011. "It drives home the threat of foreign fighters. What happens when they go home?"