One man, so many dead.
Omar Mateen opened fire at Pulse, a gay nightclub in Orlando, early Sunday — leaving 49 people dead and dozens injured. It was the worst mass shooting in modern U.S. history — and the most deadly act of terror in the country since 9/11.
Mateen died in a shootout with police, leaving the world to wonder why he did it.
Slowly, a picture of him is emerging.
Who was Omar Mateen?
Mateen, 29, was born in New York to Afghan immigrants described by one family friend as loving, close-knit and "very respectful" of America. His clan ended up in Florida, where he attended Indian River State College in Fort Pierce, a two-hour drive south of Orlando. He graduated with an associate of science degree in criminal justice technology in 2006, and later got a job as a private security guard. He was fascinated with law enforcement, people who knew him said.
He was married twice, and was the father of a 3-year-old boy.
One of his jobs was at the St. Lucie County Courthouse — as a private security officer, not as a member of the county Sheriff's Department, officials said. He worked security at PGA Village in Port Saint Lucie, where Aurelia Kennedy, a frequent visitor, became friendly with him. She told NBC News that Mateen decorated his car with a Marine-themed license plate and stickers, which led her to mistakenly believe he was a Marine.
He was friendly, she said, but seemed more withdrawn in recent days, she said.
Related: Orlando Gunman Omar Mateen 'Cool and Calm' During Negotiations
Did Mateen have a criminal record?
Mateen does not appear to have any criminal convictions. But in recent years he attracted authorities' attention.
In May 2013, co-workers at the courthouse reported Mateen had claimed he had family connections to Al Qaeda and had relatives who were in Hezbollah, two groups that are opposed to ISIS, FBI Director James Comey said. He also told colleagues that he hoped law enforcement would raid his home and assault his wife and child "so he could martyr himself," Comey said.
The FBI investigated, using informants, recording conversations with Mateen, and interviewing him twice, Comey said. Mateen admitted making the statements to his colleagues, but said it was because he was angry they were teasing him for being Muslim. The probe ended in May 2014.
Related: Omar Mateen, U.S. Suicide Bomber Tied to Fort Pierce, Florida
Two months later, in July 2014, Mateen's name came up during an investigation of a Fort Pierce man, Moner Mohammad Abu-Salha, a Palestinian-American who carried out a suicide attack in Syria on behalf of the Al-Nusra front, according to The Associated Press. Comey confirmed that account, but didn't identify the bomber, saying the two men knew each other "casually" from attending the same mosque. An investigation determined there were no ties of any consequence, Comey said.
How did Mateen get his guns?
Since Mateen worked as a security guard, he had a license to buy weapons, and he legally purchased the handgun and AR-15-style rifle used in the Pulse massacre days before the attack, authorities said.
What motivated Mateen to kill so many people?
Investigators are still trying to figure that out. But there are a lot clues.
Noor Mateen, the current wife of Omar Mateen the gunman in the Orlando shooting massacre tried to persuade him not to carry out the deadly rampage, FBI authorities tells NBC News.
She drove him to the Pulse nightclub which he wanted to see in advance and was with him when he bought a holster. Several officials say she is cooperating with investigators.
Since the shooting attack, she has been staying at the home of Mateen's father in Port Saint Lucie, Florida.
His first wife, Sitora Yusufiy, said she met him online and married him quickly in 2009, but left him within months of their marriage, which finally ended in divorce in 2011. She said he was prone to violent rages in which "he would express hate toward everything," she said. He beat her and isolated her from her family, who persuaded her to flee, she said.
She described him as being mentally ill. But he didn't show any signs of radicalization back then, she said.
Related: Gunman Omar Mateen Described as Belligerent, Racist and 'Toxic'
A former colleague at the security firm G4S Secure Solutions said Mateen often showed up for work early and was fascinated by law enforcement. But he also had a short emotional fuse and went on hateful rants, the co-worker, Daniel Gilroy, said. "He had anger management issues. Something would set him off, but the things that would set him off were always women, race or religion." Gilroy said he requested a transfer to stay away from Mateen.
During his rampage, Mateen spoke with a 911 dispatcher three times, Comey said, pledging loyalty to ISIS but also the brothers who bombed the 2013 Boston Marathon and the Al Nusra bomber. Mateen called the brothers his “homeboys," according to Hank Shaw, who runs the FBI's Boston office.
Given Al Nusra's rivalry with ISIS, Comey said it was difficult to determine Mateen's motives or loyalties.
How Mateen became influenced by radical Islam — and whether he had been in touch with any terrorists overseas — remains unclear.
President Obama on Monday morning said there was no clear evidence Mateen was directed or part of a "larger plot."
Federal officials said Mateen traveled to Saudi Arabia in 2011 and the United Arab Emirates in 2012. The Saudi foreign ministry said Mateen was in Saudi Arabia during both trips, to perform a Muslim pilgrimage called Umrah. But U.S. authorities say they want to know more about those trips.
Mateen's father, Sediqque Mir Mateen, said he saw his son frequently but noticed nothing alarming. Recently, during a visit to Miami, Omar Mateen became upset when he saw two men kissing, Sediqque Mir Mateen said. But Mateen did not appear unhinged or violent, his father said.
Related: Omar Mateen's Father Apologizes After Orlando Pulse Massacre
"He got enough attention from the family, good education, good life and with a good job. So I am as shocked as you are," he told NBC News.
Hours before the massacre, Mateen stopped by his parents' house. He said hello and left, Sediqque Mir Mateen said. He seemed calm.
"I wish he was alive so I could ask him that question. One question: Why did you do it?"
