As Moner Mohammad Abusalha 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 e-mails indicating he was in Jordan, possibly caring for the wounded but far from the fight. “I’m here now, doing fine,” the e-mails 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 Abusalha had crossed into Syria, but they had scant intelligence on his activities there or associations with an al-Qaeda affiliate until he appeared last week in an online martyrdom video.

The inability to track Abusalha 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 that 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.

Moner Mohammad Abusalha is an American who apparently embraced violent Islam. (Seminole State College of Florida/via AP)

“It’s a bit of a black hole,” one U.S. counterterrorism official said. “We don’t have a lot of collection there.”

U.S. officials described Syria as a daunting environment for espionage. The CIA pulled its people out of Syria when the U.S. Embassy was closed as the conflict moved toward civil war. There are also legal impediments to tracking U.S. citizens or monitoring their communications.

Amid estimates that as many as 12,000 foreigners have flocked 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.

Abusalha’s demise was greeted by some U.S. counterterrorism officials with a measure of relief that a U.S. citizen and willing suicide bomber had carried out his attack far from the country where he lived.

The outcome was seen as evidence that Jabhat al-Nusra, the al-Qaeda-linked group that asserted responsibility for the attack, remains focused on ousting Syrian President Bashar al-Assad rather than taking advantage of an American recruit to launch a plot against the West.

Even so, U.S. officials acknowledged that the full extent of Abusalha’s transformation — his grooming by Jabhat al-Nusra and willingness to take part in a suicide attack — might have eluded U.S. agencies even if they were in a position to spot any attempt to leave Syria.

A U.S. law enforcement official said that as many as a dozen Americans who have traveled to Syria since the outset of war there have been investigated or scrutinized since returning to the United States. Among the Americans who have traveled to Syria are prison and online converts to the cause, according to a U.S. official.

Abusalha was not known to have ever espoused violence toward the United States or been linked to any terrorist plot, meaning there may have been no cause to detain him if he had chosen to return. His death, against the backdrop of that prospect, has heightened concern.

“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?”

Details surrounding Abusalha’s background, his departure from the United States and his apparent embrace of violent Islam remain murky. Relatives in Florida did not respond to repeated requests for comment. Neighbors and family friends offered only fragments of information about the bomber and his family.

Husainy, who said he spoke to Abusalha’s father on Friday, said the family was stunned by the death. “It came as a shock,” Husainy said.

Abusalha was known as a devout Muslim among his friends and high school classmates. His mother often wore a traditional Islamic head scarf in their Vero Beach neighborhood, according to Mark Hill, who said he has lived around the corner from the family since 2006.

“They’re very good, very happy, very friendly people,” Hill said. Abusalha was an avid basketball player, often seen dribbling around the neighborhood.

Abusalha attended classes at Indian River State College in Fort Pierce in 2010 and 2011, taking preparatory classes for physical therapy, according to college officials. He later transferred to Seminole State College of Florida and was last enrolled in 2012.

Abusalha was suspended in high school after fighting with classmates who made fun of his mother’s Islamic clothing, neighbors said. A close friend said he would take part in religious events including “fast festivals,” where he would forgo food for days

Cecilio Moreno, a Port St. Lucie resident, met Abusalha four years ago at the Islamic center they both attended in Fort Pierce. Moreno, 41, said Abusalha stopped attending services at the mosque two years ago.

“He was really, really happy . . . always smiling,” she said.

When he left the United States within the past several months, U.S. officials said, his departure was abrupt, and he appears to have kept his itinerary a secret.

Only weeks later did he send a note to his family saying he was in Jordan, his father’s ancestral home. The family continued to get cryptic e-mails saying, “I’m fine, I’m okay. I’m alive,” Husainy said.

Abusalha surfaced intermittently on social networking sites, leaving clues to his evolving beliefs, according to the SITE Intelligence Group, which monitors online Islamist postings.

Images loaded to a Facebook account in 2012 and 2013 show green birds, a motif associated with Islamist martyrs, according to the group.

Those online postings might have drawn the attention of U.S. law enforcement agencies, which monitor hundreds of Web sites as part of an effort to identify and track foreign fighters.

It took days for U.S. officials to confirm that Abusalha had taken part in the bombing depicted on a video posted last week by Jabhat al-Nusra. The footage includes an image of Abusalha smiling and cradling a cat, a pose used by the al-Qaeda affiliate to send a reassuring message to would-be recruits. The video shows fighters packing a heavily armored truck, then rolling toward a hilltop compound that is destroyed moments later in a massive explosion.

Earlier this year, Director of National Intelligence James R. Clapper Jr. testified that Jabhat al-Nusra has “aspirations for attacks on the homeland” and that al-Qaeda groups have established camps “to train people to go back to their countries.”

There have been no known efforts by groups in Syria to attack the United States. But plots elsewhere have been attributed to spillover from Syria. An alleged French jihadist who had been in Syria was arrested after killing three people in a shooting at a Belgian Jewish museum May 24.

France has seen as many as 700 of its citizens travel to Syria, more than any other European country. Thousands are believed to have come from nations in the Middle East and Africa, including Saudi Arabia, Tunisia and Morocco.

Bruce Riedel, a former senior analyst at the CIA, wrote in a recent article that Syria is “all but certain to be an even larger factory for extremism than Afghanistan.”

Rodriguez reported from Port St. Lucie. Julie Tate in Washington contributed to this report.