A few key revelations about his past came to light hours after the 29-year-old was identified as the shooter who took hostages at the Pulse nightclub in Orlando early Sunday.
He was subsequently killed by a SWAT team.
An ex-co-worker told NBC News that Mateen was racist, belligerent and "toxic."
His father told NBC News his son was enraged after recently seeing a same-sex couple kissing in front of his family, an event that could have set him off.
In 2013, Mateen was interviewed twice by federal agents after co-workers reported that he made "inflammatory" comments to them about radical Islamic propaganda. The following year, the FBI looked at him again because of ties with an American who traveled to the Middle East to become a suicide bomber.
Law enforcement sources told NBC News he swore allegiance to the leader of ISIS, Abu Bakr al-Baghdadi, in a 911 call moments before the rampage at Pulse.
There's no indication that Mateen was in touch with terrorists overseas or that the attack was directed by someone else, a law enforcement officials told NBC News. Nor is there evidence that anyone helped or encouraged him, several officials said.
Investigators are looking into whether religious extremism motivated the attack.
Mateen, who lived roughly two hours south of Orlando in Fort Pierce, worked as a security guard.
Daniel Gilroy, a co-worker at G4S Secure Solutions — formerly known as Wackenhut — told NBC News that Mateen was a "very conscientious" employee who was often early for work and was fascinated by law enforcement.
He was also loud, "very excitable" and racist, Gilroy said.
"He was scary in a concerning way," he said. "And it wasn't at times. It was all the time. He had anger management issues. Something would set him off, but the things that would set him off were always women, race or religion. [Those were] his button pushers."
Working with Mateen was so difficult that Gilroy said he requested a transfer.
"I needed to be out of that situation," he said. "I described it as being toxic."
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Mateen "always referred to every other race, religion, gender in a derogatory way," Gilroy added. "He did not like black people at all. That was mentioned once or twice, but more so was women. He did not like women at all. He did like women in a sexual way, but he did not respect them."
His father told NBC News that his son was affected by a recent incident involving two men showing each other affection.
"We were in downtown Miami, Bayside, people were playing music. And he saw two men kissing each other in front of his wife and kid, and he got very angry," Mateen's father, Seddique Mir Mateen, said Sunday. "They were kissing each other and touching each other, and he said: 'Look at that. In front of my son, they are doing that.' And then we were in the men's bathroom, and men were kissing each other."
"We are saying we are apologizing for the whole incident," the elder Mateen said. "We weren't aware of any action he is taking. We are in shock like the whole country."
Driving the point home that religion was a consideration in the mind of investigators, officials brought a member of the Muslim community to speak at a news conference.
Mateen didn't appear to have any direct ties with ISIS, sources said, although he was a follower of ISIS propaganda and referred to the Tsarnaev brothers, who carried out the Boston Marathon bombings in 2013, at the scene of the shooting.
But while law enforcement delves into what may have radicalized Mateen, who was born in New York and lived in Florida for at least the past decade, his family believes he was fueled by pure hate against the LGBT community.
Police did not explicitly say Mateen was Muslim, but Islamic groups put out statements denouncing the carnage.
"We condemn this monstrous attack and offer our heartfelt condolences to the families and loved ones of all those killed or injured. The Muslim community joins our fellow Americans in repudiating anyone or any group that would claim to justify or excuse such an appalling act of violence," Rasha Mubarak, Orlando regional coordinator for the Council on American-Islamic Relations, said in a statement.
Seddique Mir Mateen said his son was a husband and father to a 3-year-old son. He worked in security and attended Indian River State College. A spokeswoman said he got an associate of science degree in criminal justice technology in 2006.
Police say he used a handgun and AR-15-type rifle in the shooting spree.
The U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives officials tweeted Sunday that Mateen legally bought the firearms within the last week.
Records also show that he had filed a petition for a name change in 2006 from Omar Mir Seddique to Omar Mir Seddique Mateen.
His ex-wife, Sitora Yusufiy, who lives in Colorado, told reporters that Mateen was prone to violent behavior and beat her.
Besides abusing her physically, he also isolated her from her family, who worked to persuade her to leave the marriage, she said.
"When he got in his tempers, he would express hate toward everything," Yusufiy said.
Editor's note: An earlier version of this article said the gunman killed at least 50 people. Authorities have since said that there were 49 shooting victims, plus the gunman.
Pete Williams is an NBC News correspondent who covers the Justice Department and the Supreme Court, based in Washington.
Tracy Connor is a senior writer for NBC News. She started this role in December, 2012. Connor is responsible for reporting and writing breaking news, features and enterprise stories for NBCNews.com. Connor joined NBC News from the New York Daily News, where she was a senior writer covering a broad range of news and supervising the health and immigration beats. Prior to that she was an assistant city editor who oversaw breaking news and the courts and entertainment beats.
Earlier, Connor was a staff writer at the New York Post, United Press International and Brooklyn Paper Publications.
Connor has won numerous awards from journalism organizations including the Deadline Club and the New York Press Club.
She lives in Brooklyn, N.Y.
Erik Ortiz is an NBC News staff writer focusing on racial injustice and social inequality.
Stephanie Gosk is an NBC News correspondent based in New York City. She contributes to “Nightly News with Lester Holt,” “TODAY” and MSNBC.