WASHINGTON >> When a young American man from coastal Florida drove a truck packed with explosives into a hilltop restaurant in Syria in May 2014, FBI agents scoured his online postings and interviewed his contacts in Florida in a scramble to determine who, if anyone, might try to launch a similar attack inside the United States.

One of the people they spoke to was Omar Mateen, a young security guard from a nearby town who had attended the same mosque as the suicide bomber and who had been on terrorism watch list for incendiary comments he once made to co-workers at a local courthouse. But the FBI soon ended its examination of Mateen after finding no evidence that he posed a terrorist threat to his community.

That hopeful conclusion was upended in a bloody spasm of violence early Sunday, when Mateen murdered dozens of people at a nightclub in Orlando, Fla., before being killed by police officers who stormed the club to end the standoff. The horrific events at the Pulse nightclub left 49 dead and have left family members, neighbors and federal investigators trying to piece together clues about what might have led Mateen, 29, to carry out such unspeakable violence.

The government investigation could take months, but an early examination of Mateen’s life reveals a stew of contradictions. He was man who could be charming, loved Afghan music and enjoyed dancing at family ceremonies, but he was also violently abusive. Family members said he was not overly religious, but he was rigid and conservative in his view that his wife should remain mostly at home. The FBI director said Monday that Mateen had once claimed ties to both al-Qaida and Hezbollah — two radical groups violently opposed to each other.

Investigators now face the question of how much the killings were the act of a deeply disturbed man, as his former wife and others described him, and how much he was driven by religious or political ideology. Whatever drove him to carry out the shootings, his actions highlight the difficulty for the U.S. government in trying to address a new style of terrorism — random acts of violence that may have been at least partly inspired by the Islamic State but were not directed by the group’s leaders.

Unlike al-Qaida, which favors highly organized and planned operations, the Islamic State has encouraged anyone to take up arms in its name, and uses a sophisticated campaign of social media to inspire future attacks by unstable individuals with little history of embracing radical Islam. President Barack Obama said Monday that there was no evidence that the Islamic State directed Sunday’s attack, which would make Mateen’s case part of a pattern of domestic radicalization.

U.S. officials have said that those under surveillance in the United States for possible ties to the group usually have little terrorism expertise or outside support, which makes thwarting an ISIS-inspired attack less like stopping a traditional act of terrorism and more like trying to prevent a shooting at a school or movie theater.

The son of Afghan immigrants, Mateen was born in New York in 1986, moved to Florida with his family in 1991 and spent his early years there living a relatively uneventful life in the Port St. Lucie area on the state’s east coast. He made friends as a child at a local mosque, and built friendships during slumber parties, basketball games and playing video games. He bounced between jobs in high school and college. In court documents connected to a 2006 name change — from Omar Mir Seddique to Omar Mir Seddique Mateen — he said he had held eight jobs in about four years, from a grocer to a salesman at a computer store.

He earned an associate degree in criminal justice technology from Indian River State College in 2006, the same year he began working for the Florida Department of Corrections at a facility just west of Port St. Lucie.

He left that job six months later, and within six months he had found work with G4S, a large private security company that has won large government contracts for work both in the United States and abroad. He was assigned to protect at least two properties during his years at the firm: PGA Village golf club and the St. Lucie County Courthouse complex.

He met his future wife, Sitora Yusufiy, on MySpace in 2008. Both were on the site looking for love and eventually marriage, and she was drawn to him because of his alluring and funny messages.

During an interview Monday at her home in Boulder, Colo., Yusufiy said he seemed perfect — American enough for her free spirit and Muslim enough to please her traditional family.

“This man was a simple, Americanized guy that was also from my culture. And, you know, had the same religion,” she said. “So I was like, OK, this could potentially satisfy my parents.”

She moved to Florida, and they married in a quiet courthouse ceremony in 2009, but the short-lived marriage was marred by violence and isolation. She had no friends or family in Florida, and Mateen preferred that she stay in the house.

She said he sometimes returned from work angry and agitated, including one night when she fell asleep on the floor waiting for him to return home.

“All I remember is being woken up by a pillow being taken from under my head,” she said. “I hit my head on the ground and then he started pulling my hair.”

“He almost killed me,” she said. “Because he started choking me. And I somehow got out of it and I tried to tackle him.”

The couple separated within a year, and in 2011 Mateen filed for divorce. In the court filing, Mateen said the marriage was “irretrievably broken.” He did not elaborate.

He came to the FBI’s attention in 2013, when some of his co-workers reported that he had made inflammatory comments claiming connections to overseas terrorists, and saying he hoped the FBI would raid his family’s home so he could become a martyr.

The FBI opened an investigation and put Mateen on a terrorist watch list for nearly a year.

James Comey, the FBI director, said during a news conference Monday that agents used various methods to investigate Mateen, including sending an undercover informant who made contact with the suspect, wiretapping his conversations and scrutinizing his personal and financial records.

They also sought help from Saudi intelligence officials to learn more about his trips to the kingdom in 2011 and 2012 for the Umrah, a sacred pilgrimage to Mecca made by Muslims. More than 11,000 Americans make pilgrimages to Mecca each year, and Comey said the FBI found no “derogatory” information about his trips.

During interviews with FBI agents, according to Comey, Mateen said he had made the incendiary remarks “in anger” because his co-workers had ridiculed his Muslim background and he wanted to scare them. The FBI closed its investigation and took him off the terrorist watch list.

But two months later, in July 2014, his name resurfaced in connection with the young man from coastal Florida, Moner Mohammad Abusalha, who had traveled to Syria and carried out the suicide bombing at the hilltop restaurant. During the course of that investigation, FBI agents learned that the two men had attended the same mosque and knew each other “casually,” Comey said.

The FBI interviewed Mateen a third time, but determined that his ties to the suicide bomber were not significant. The bureau had no further contact with Mateen.

Comey defended the work of his agents, although the bureau’s handling of the case is likely to be the subject of scrutiny and criticism in the coming weeks.

Still, cases such as these rankle FBI counterterrorism agents, who believe they draw criticism for any choices they make — either for leaving cases open too long, or for closing cases that don’t seem to have enough evidence.

Don Borelli, a retired FBI counterterrorism supervisor in New York, said there was a danger in criticizing agents who close investigations for lack of evidence.

“Can we allow people’s futures to be affected if there is no proven basis for it? That’s the flip side to all this,” he said.

Sally Yates, the deputy attorney general, told reporters Monday that the Justice Department might look to adopt new procedures that would alert counterterrorism investigators if someone who had been on a terror watch list tried to buy a gun.

Mateen bought the two weapons used in the attack just this month, officials said. “One would have liked to have known about it,” Yates said.

Federal investigators are now left to sift through disparate clues in search of any clear motive for Sunday’s killings.

The Islamic State has tried to turn the bloody event into a propaganda coup, and on Monday the group’s daily news bulletin boasted about the great victory carried out by “our brother, Omar Mateen.”

In a separate online message, one ISIS member in Yemen compared the attack to the Battle of Badr in A.D. 624, a military victory for Mohammed’s army referenced in the Quran.

Mateen’s father, Seddique Mir Mateen, was unequivocal Monday that his son had committed an “act of terrorism.” But the elder Mateen and other family members said they were still puzzled why a young man who had never been particularly religious is now being tied to the Islamic State’s murderous ideology.

They said that at this point they can find no easy explanations.

“Why did he do this?” his father asked. “He was born in America. He went to school in America. He went to college — why did he do that?”

“I am as puzzled as you are.”