When an American suicide bomber drove a truck packed with explosives into a government building in Syria in May, he had just returned from several months in his Florida hometown, where U.S. officials say he tried to recruit some of his friends for Syria's violent revolution.
At least one of the men he tried to recruit told the FBI, but by then, Moner Mohammad Abusalha was already headed back to Syria and his suicide bombing mission.
The threat raises questions about how Abusalha, who grew up a basketball-obsessed teenager in a gated community in Florida, ended up as a suicide bomber. And, could he -- should he -- have been stopped by U.S. law enforcement?
Said a senior law enforcement official, "The Abusalha case is an object lesson in how difficult it is to identify self-radicalized people in the U.S., and to track people who go from the U.S. to Syria and return. This difficulty has been a repeated subject of publicly expressed concern for months."
The revelation came after the al-Qaeda-backed group Abusalha died for -- the al-Nusrah Front -- released a 31-minute video that amounts to his last will, common for suicide bombers. In it, he wishes his family well, tells his mother, "I love, I love you, Mom. Stay strong for Allah," and is shown ripping, chewing and setting fire to his U.S. passport.
The sequence of events that led up to the bombing, one of several on the last weekend in May, is murky. U.S. officials admit they don't have all the details of Abusalha's travels. His journey began more than a year ago, when after wandering around in the Middle East he wound up in Syria. In his video will, he says the Syrian rebels clothed and fed him and trained him as a fighter.
After that training, he returned to Ft. Pierce, Florida for several months in 2013. It was during that visit, say law enforcement officials, that he tried to recruit others to go back to Syria with him. They refused. The official tells NBC News that one of those people contacted the FBI, and that's how the U.S. became aware of him as he returned to Syria the second time.
Law enforcement officials admit U.S. intelligence, including the FBI, did not know about Abusalha's radicalization and his first visit to Syria or that he had returned to the U.S. to recruit. They were also unaware, say intelligence officials, of his plan to kill himself.
Three days after the May 25 bombing, al-Nusrah tweeted out word of his role in the suicide bombing.
"Abu Hurayra Al-Amriki performed a martrydom operation in Idlib, Jabal Al-Arba'een. May Allah accept him," it said.
The tweet included an image of Abusalha holding a cat, as well as images of the bombing al-Nusrah said he had carried out. "Amriki" means "American" and one U.S. official pointed out that "Abu Hurayra" is Arabic for "father of the kitten" or "of the kitten," and was the name adopted by a cat-loving companion of Muhammad, Islam's founder. A video showed him loading a vehicle with explosives. Al-Nusrah claimed this was the vehicle he had detonated in the attack.
Then, on Monday, al-Nusrah released his will. In it, Abusalha attacks American culture and issues a threat.
"I lived in America. I know how it is," he says. "You have all the fancy amusement parks, and the restaurants, and the food, and all this crap and cars. You think you're happy? You're not happy."
"You think you're safe where you are, in America or Britain," he adds. "You think you are safe. You are not safe."
Michael Leiter, former director of the National Counterterrorism Center and an NBC News analyst, says it appears no red flags were missed.
"I haven't seen anything that suggests that the FBI and Homeland Security missed something with this individual initially traveling to Syria. And once they were tipped off after he came back to the United States, I believe they tried to track him. But it is very hard to track thousands of miles away any single individual... who he's fighting with, where he's going, and whether he poses a threat."
Leiter and others in intelligence and law enforcement are concerned about what some are calling "vacationing terrorists," those who go and fight in Syria, then return home to American soil. Officials say at any one time, around a dozen U.S. nationals are fighting in Syria with radical groups and that somewhere between 70 and 100 Americans have made the trip to Syria to fight for al-Nusrah or ISIS, the group that recently established an Islamic caliphate in Reqqah, Syria, covering a stretch of territory between eastern Syria and western Iraq.
There's at least one "very active" federal investigation, say U.S. officials. In Minnesota, the FBI is investigating a "handful of people" from the greater Minneapolis area who went to Syria and then returned to the States.
"Westerners traveling to Syria are the single biggest threat the counterterrorism community faces today," said Leiter. "Not al Qaeda in Pakistan... this is the group that more than a thousand Westerners have traveled to, are fighting for, are getting training from. And the challenge is identifying them and then stopping them when they come back if they're actually plotting attacks back in the United States."
Both the FBI and the Department of Homeland Security are considering rule changes to more stringently screen those traveling in and out of the U.S. to try to identify others who pose a threat.