The Post's Greg Miller and Adam Goldman discuss the life and violent death of 22-year-old Florida native Moner Mohammad Abusalha, the first American to commit a suicide bomb attack in Syria. (Davin Coburn/The Washington Post)

There were no U.S. air marshals watching the newly clean-shaven passenger on the transatlantic­ flight, no FBI agents waiting for him as he landed in Newark in May 2013 after returning from Syria’s civil war.

As the 22-year-old Florida native made his way through a U.S. border inspection, officers pulled him aside for additional screening and searched his belongings. They called his mother in Vero Beach to check on his claim that he had merely been visiting relatives in the Middle East. But when she vouched for him, U.S. officials said, Moner Mohammad Abusalha was waved through without any further scrutiny or perceived need to notify the FBI that he was back in the United States.

Earlier this year, after returning to Syria, Abusalha became the first American to carry out a suicide attack in that country, blowing up a restaurant frequented by Syrian soldiers on behalf of an al-Qaeda affiliate. His death May 25 was accompanied by the release of a menacing video. “You think you are safe where you are in America,” he said, threatening his own country and a half-dozen others. “You are not safe.”

It was a warning from someone who had been in position to deliver on that threat. By then, Abusalha­ had made two trips to a conflict zone seen as the largest incubator of Islamist radicalism since Afghanistan in the 1980s. Between those visits he wandered inside the United States for more than six months, U.S. officials said, attracting no attention from authorities after their brief telephone conversation with his mother.

His movements went unmonitored despite a major push by U.S. security and intelligence agencies over the past two years to track the flow of foreign fighters into and out of Syria. At the center of that effort is a task force established by the FBI at a classified complex in Virginia that also involves the CIA and the National Counterterrorism Center.

A video released in July by Manara al-Bayda, the media arm of the al-Qaeda affiliate Jabhat al-Nusra, shows Moner Mohammad Abusalha, who in May became the firs t American suicide bomber in Syria. (Manara al-Bayda via AP)

Despite that expanding surveillance net and more than a dozen prosecutions in the United States, the outcome for Abusalha depended more on the priorities of his al-Qaeda handlers than U.S. defenses. FBI officials involved in the case said it exposed vulnerabilities that can be reduced but not eliminated.

“It is extremely difficult for the FBI to identify individuals in the U.S. who have this kind of goal,” said George Piro, special agent in charge of the FBI’s Miami field office, which led the Abusalha investigation. “It requires a loved one or really close friend to note the changes. . . . The family has to intervene.”

Abusalha is counted among the 100 or so Americans who have traveled to Syria or attempted to do so, a figure cited repeatedly by senior U.S. officials in ways that suggest there is precision in their understanding of who and where those people are.

In reality, officials said, the total has risen to 130 or more, and it includes individuals about whom only fragments of information are known. The clearest cases­ involve U.S. citizens arrested by the FBI before they depart. But other cases are incomplete, based on false names or partial identities assembled from references on social media or U.S. intelligence sources.

Even the estimate of 130 is low, according to U.S. officials who said there are undoubtedly Americans in Syria and Iraq who have not surfaced. Abusalha was part of that invisible category until shortly before he recorded his farewell videos and stepped into the cab of an armored dump truck packed with explosives.

FBI Director James B. Comey recently warned of such blind spots. “Given the nature of the traveler threat, I don’t sit with high confidence that I have complete visibility,” Comey said in a briefing at FBI headquarters. “Who are we missing who went and came back? And, obviously, who are we missing who is in the midst of trying to go?”

How big a threat?

Aspects of Abusalha’s case make it tempting to play down the threat he posed. A wayward youth who cycled through three Florida colleges without earning a degree, he appears to have stumbled into the ranks of al-Qaeda’s affiliate in Syria rather than being recruited, let alone groomed as a high-level operative.

Map: Flow of foreign fighters to Syria

This account is based on interviews with U.S. officials and family members who provided the most detailed reconstruction to date of Abusalha’s case. Many spoke on the condition of anonymity in order to discuss aspects of the investigation and family matters.

Abusalha was filmed shredding and eating his U.S. passport, destroying a document that would have been critical if his al-Qaeda handlers had any impulse to employ him in a plot against the West. By the time he killed himself, he was already on the U.S. no-fly list, added to that terrorism database after the FBI fielded a tip that Abusalha had gone to Syria.

Still, there are other reasons to regard the Abusalha case as a close call. Most significant, he appears to have joined the ranks of an al-Qaeda affiliate known as Jabhat al-Nusra that was forming a clandestine cell of veteran al-Qaeda operatives charged with developing plans to attack the United States and its allies.

The cell, known as the Khorasan­ group, was established in part to take advantage of the influx of fighters with Western passports. It is not clear whether Abusalha crossed paths with Khorasan members, but a barrage of U.S. missile strikes aimed at the group last month struck targets on the western outskirts of Aleppo — a short distance from where Abusalha detonated his truck bomb.

U.S. counterterrorism officials are still debating the significance of al-Qaeda’s decision to use Abusalha in an attack on the forces of Syrian President Bashar al-Assad, rather than aiming him at a Western target.

“One possibility is they have so many guys they’re just expendable,” a senior U.S. counterterrorism official said. “More likely, right now they’re just focused on Assad.”

Americans account for a tiny fraction of an estimated 15,000 fighters who have flocked to Syria from other nations. The majority are from elsewhere in the Middle East, but at least 700 have traveled or attempted to travel from France, with an additional 400 from Britain and 250 from Germany.

Still, U.S. officials described the domestic security challenge as daunting, in part because the potential danger is so dispersed. More than 250,000 Americans went in and out of Turkey last year alone, U.S. officials said, part of a pool of more than 2 million travelers who arrive in the United States annually from countries surrounding the conflict in Syria.

When analysts at the National Counterterrorism Center plotted known addresses of those who had gone to Syria or tried to on a map of the United States, they found no clumps where the FBI could concentrate its efforts, just scattered dots.

Drifting through life

The FBI employs numerous means of spotting would-be jihadists­: using networks of informants and undercover agents in Muslim communities; tracking Internet chat rooms and social media sites where militants congregate; and relying heavily on the National Security Agency to monitor messages between Americans and suspected terrorists overseas.

Abusalha never surfaced on any of those screens. “He hit none of the tripwires,” one senior U.S. official said.

Abusalha did not seemed particularly concerned with avoiding them. His initial departure for Syria took place in late January 2013. He told his parents that he was heading to a mosque in Fort Pierce, Fla., and left his champagne-colored­ Chevrolet Malibu in the parking lot, with the keys still in the ignition.

Abusalha was living with his parents at the time. His father, a Palestinian, and his American mother, a convert to Islam, operated a small market. The family endured occasional economic setbacks, including the foreclosure of their home about five years ago, as well as episodes of discrimination. Neighbors recalled Abusalha being suspended from high school after he fought with classmates who mocked his mother for wearing a hijab.

After high school, Abusalha drifted through jobs and half-hearted attempts at college, struggling to find his footing in a life that he railed against in his suicide videos.

“You have all of fancy amusement parks and the restaurants and the food and all this crap and the cars. You think you’re happy,” he said. “This life sucked. . . . All you do is work 40, 50, 60 hours a week, and then you go waste it on garbage, and then you do the same thing.”

Always religious, he gradually embraced ever stricter interpretations of Islam. He told one friend that he had dedicated himself to pursuing the “straight path” of the Koran, meaning a life free from sin. He avoided restaurants where alcohol was served and traveled to take part in “fast festivals” at which participants abstained from eating for days.

His social circle at the Islamic Center of Fort Pierce contracted until it consisted mainly of his older brother, Mahrous, the family of the imam, and a 26-year-old convert to Islam, Robert Stewart, who seemed equally drawn to the most extreme versions of the faith.

The Washington Post identified Stewart in interviews with Abusalha’s family and friends. They said they suspect that he played a critical role in solidifying Abusalha’s religious views, and in backing his decision to fight in Syria.

Stewart did not respond to requests for an interview sent though family members, e-mail, Facebook and phone numbers that he is thought to have used.

“The two were like-minded,” a senior U.S. official said. “It’s not clear who radicalized whom.”

After leaving his car at the mosque, Abusalha made his way to Orlando and caught a flight the next day that took him through Frankfurt, Germany, and Amman, Jordan, before he arrived in Ankara, the Turkish capital.

When Abusalha didn’t come home, his parents called the imam at Fort Pierce, and he found the car outside. Abusalha’s mother and sister eventually discovered an itinerary of his trip on the family’s computer.

Relatives and friends in Florida offered conflicting accounts of their awareness of Abusalha’s intent. Abusalha did not share with his mother all of the details about his travels when he called to announce that he was heading to Amman to stay with family. One relative who spoke to Abusalha in Amman recalled the young man saying that he had met with members of the moderate Free Syrian Army and became disillusioned because they were not pushing to establish an Islamic state.

In any case, Abusalha’s first stay in Syria was short. In late February 2013, he left for Jordan, where his grandparents lived. From there, he and his grandmother made a pilgrimage by bus to the Islamic holy cities Mecca and Medina, a trip that lasted 12 days. Then, on May 12, he flew from Amman to Paris to Newark.

When Michelle Abusalha was called by a U.S. Customs and Border Protection officer about her son, she confirmed that he had been visiting relatives in Jordan. If she knew or suspected that Abusalha had been in Syria, she did not share that information.

Abusalha’s encounter with Customs and Border Protection appears to be a critical moment. Even without forthright answers from the family, Abusalha was a military-aged male coming back from a country on the edge of a war zone. Had the bureau been notified, agents probably would have opened at least what is known as an assessment, questioning the family in Florida and Abusalha himself.

But U.S. officials familiar with the airport screening defended the CBP officers’ actions. Abusalha’s travel records showed multiple trips to Jordan dating to 2005, a senior Homeland Security official said. There was no entry on Abusalha in any FBI or counterterrorism database. Nothing in his behavior or belongings betrayed any tie to the conflict in Syria.

“We could find no d-rog,” the senior Homeland Security official said, using jargon for “derogatory” information on a traveler. “Just traveling to Jordan isn’t a reason to open up a full investigation” or notify the FBI, the official said, otherwise “we’d be calling them millions of times a year.”

“If the FBI was looking for this person and had a case on him, we absolutely would have notified the FBI,” the official said.

The next six months for Abusalha trace the path of someone unsure of his next move. He moved back in with his family in Vero Beach and looked for jobs while also helping out his father, a devout man who was born in Nablus in the West Bank, at his family’s Middle Eastern grocery store in Melbourne.

Abusalha began to regrow the beard he had shaved off for his flight home from Jordan, but family members said he showed no outward signs of being under the sway of al-Qaeda or extreme Islam. Then, two months after coming home, he was gone again, departing in the middle of the night for San Antonio, where he reunited with Stewart. The two struggled to find jobs and places to stay. In one of his videos, Abusalha described being tempted to forage for food in Dumpsters.

In September 2013, Abusalha saw his mother one last time when she made a trip to see relatives in Houston. Family members said she was troubled by his appearance and concerned that he seemed exhausted and withdrawn. The family persuaded him to stay in Houston, hoping, as one said, “to get him away from the Islamic radical thinking.”

Instead, Abusalha appears to have begun making plans for a return trip to Syria, this time with Stewart. In October 2013, Abusalha stole credit cards from a relative who was trying to help him and disappeared again. “If we had known, we would have tied him up with rope,” one of his relatives said.

In mid-November, records show, Abusalha flew from San Antonio to Orlando. He and Stewart appear to have spent the following week making final preparations for their departure overseas, although Abusalha seems to have misjudged the commitment of his companion in jihad.

The two arrived at the airport in West Palm Beach, Fla., packed for a journey that was supposed to take them through New York to Istanbul and Cairo — a false endpoint apparently selected to disguise their intent to break off in Turkey and make their way to Syria. Only one boarded the aircraft.

“When I was waiting for my flight, I told my friend to watch the stuff, I’m going to do ablution,” Abusalha said in one of his videos. “I came back, my bag was there, but he was gone. And I was waiting and waiting, and thinking maybe he went to get something to eat or something like this, waiting, waiting, waiting. Then, we were boarding airplane. I get on airplane, and the airplane is backing up and getting ready to fly. And that’s when I know that my friend, he backed out, glory be to Allah. It hurt me so much because I love my friend a lot, very much.”

A few days later, Abusalha called his mother. “Where are you?” she asked, according to family members. He replied: “Where I was originally.”

Grooming a suicide bomber

Abusalha disappeared deeper this time into a civil war that had come to be dominated by insurgent forces with al-Qaeda lineage. One, the Islamic State, split off from al-Qaeda this year before declaring a caliphate in northern Iraq and Syria. The other, al-Nusra­, became al-Qaeda’s designated affiliate, and it welcomed Abusalha into its ranks.

How he reached al-Nusra’s base of operations near Aleppo remains unclear. In his videos, Abusalha described arriving in Istanbul with only a few dollars in his pocket, walking miles from the airport to the city before linking up with militants at a prominent mosque. U.S. officials regard some of the claims as exaggerated but have been unable to completely reconstruct his path.

In videos and pictures released after his death, Abusalha is seen smiling and posing with a kitten. But the recordings also provide a glimpse into how he was groomed to commit suicide. He speaks reverently of the footage of other “martyrs” that he was shown by those overseeing his indoctrination, particularly one showing a “Russian sister of ours” who carried out a suicide attack in Afghanistan.

“This video that I watched from the sister inspired and touched me,” Abusalha said, reciting a religious passage that she had spoken in her own suicide video promising paradise for “that person who has spilled his blood” for Allah.

The FBI remained unaware of Abusalha’s radicalization and return to Syria until mid-December, when a source told the bureau about his plan to fight for al-Nusra­. The bureau opened a full-blown investigation and added Abusalha to terrorism databases and, eventually, to the U.S. no-fly list.

In May, in their final phone conversation, Abusalha’s mother told him to stay in Syria and find a wife. Later, she explained feeling trapped, according to family members. If he stayed in Syria, she knew that he could be killed in battle or captured and executed. But if she turned to the FBI, any prospect of her son returning home would probably also mean a prison term.

Abusalha did not tell her that he had already committed himself to a different fate. Instead, he saved his farewell message to her for a video that would be released after his death. “Mother, I love you, mom,” he said. “Stay strong for Allah.”

The camera follows the armored, explosive-laden truck until it disappears on the horizon. Moments later, an enormous explosion fills the frame.

The family learned about Abusalha’s death when they saw the footage on news networks, and they were soon inundated by calls from reporters. The FBI arrived the next day and interviewed Abusalha’s mother for four hours.

They also questioned Stewart, who was no longer welcome at the mosque in Fort Pierce, in Orlando. He later bragged to a friend that he got a free McDonald’s meal for agreeing to talk, and that “a huge burden has been lifted off me.”

The government has assembled a chronology of Abusalha’s case, but U.S. officials acknowledge that parts of that timeline, particularly in Syria, remain blurred. Since then, other Americans have surfaced in Syria, requiring new entries on the U.S. list of foreign fighters, including some with no names.

Last week, the FBI took the extraordinary step of posting a video on its Web site of a masked fighter in Syria who speaks with an American accent and urges Westerners to join the Islamic State, a group that has beheaded two Americans and two Britons in recent months. The FBI posting includes a link to a newly established tip line promising anonymity to those who provide information about Americans joining al-Qaeda offshoots in Syria.

“We’re hoping that someone might recognize this individual and provide us with key pieces of information,” Michael Steinbach, assistant director of the FBI’s counterterrorism division, says in an appeal on the bureau’s Web site. “No piece of information is too small.”

Miller reported from Washington. Julie Tate in Washington contributed to this report.