Omar Mateen — a man who had been on the FBI’s radar twice — has been identified by officials as the gunman who killed 50 people and left at least 53 injured at a gay club in Orlando, Florida, early Sunday.
Mateen, a 29-year-old American citizen born in New York, acted alone and very quickly when he opened fire with a weapon officials described as an assault rifle.
About 20 minutes before the killings, Mateen called 911 to pledge his allegiance to ISIS. He gave his full name to the operator, and mentioned the Boston bombers, according to multiple sources who spoke to law enforcement.
Mateen had purchased a "long gun" and a handgun within a week of the Orlando attack, the Bureau of Alcohol, Tobacco, Firearms and Explosives said. The weapons were purchased legally, despite Mateen being investigated for possible terrorist ties several years ago.
The FBI first became aware of Mateen in 2013, when he made threats to co-workers, "including possible ties to terrorist activities," an FBI spokesman said Sunday.
One of those co-workers, Daniel Gilroy, who worked with Mateen as a security guard, said he "complained multiple times that he was dangerous." Mateen showed anger toward black people, gay people, Jewish people and women, Gilroy told the LA Times.
"You meet bigots," Gilroy told the publication. "But he was above and beyond. He was always angry, sweating, just angry at the world."
Mateen was again investigated by the FBI in 2014, over possible ties to American suicide bomber Moner Mohammad Abusalha, the agency said.
Abusalha -- who lived Fort Pierce, Florida, as did Mateen -- was the first American to carry out a suicide attack in Syria. Mateen's contact with Abusalha was deemed "minimal" and did not pose a threat at the time, said FBI Special Agent in Charge Ron Hopper.
All FBI investigations of Mateen were eventually closed.
The gunman's father, Seddique Mateen, said his son grew upset after seeing two gay men kissing. The father later said in a Facebook video that "God himself will punish those involved in homosexuality," The Washington Post reported. He described Mateen as "a good son."
"He was not a stable person," Sitora Yusifiy, Mateen's ex-wife, told The Washington Post. Their short-lived marriage came to an end after Mateen became abusive toward her and her family had to "rescue" her, she said.
Authorities say Mateen attacked the downtown Pulse nightclub around 2 a.m. At one point, after a gun fight with an off-duty, armed officer, Mateen left the building but returned to take hostages. He was killed by police following the hourslong hostage situation.
This article has been updated with information from people who knew Mateen.