At least 50 people were murdered and 50 more people were seriously injured early Sunday morning at a gay nightclub in Orlando, Florida, making it the deadliest mass shooting in U.S. history, and the deadliest attack against the LGBTQ community in U.S. history. More information has come out about the shooter, who has been identified as 29-year-old Omar Mateen, a U.S. citizen living in Port St. Lucie who was born in America and whose family is from Afghanistan.

In an interview with NBC News, Mateen's father, Mir Seddique, said that the shooting "has nothing to do with religion," and stemmed instead from an incident a few months ago when his son "saw two men kissing in Miami" and became enraged.

"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," Seddique said. "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."

He added, "We are saying we are apologizing for the whole incident. We weren't aware of any action he is taking. We are in shock like the whole country."

The Washington Post reports that Mateen" apparently made a 911 call before the attack identifying himself and pledging allegiance to the leader of the Islamic State." Officials added that Mateen made a reference to the 2013 bombing of the Boston Marathon during this call, citing the names of Boston Marathon bombers Dzhokhar and Tamerlan Tsarnaev. Rep. Adam Schiff confirmed that local law enforcement officials said the gunman had “declared his allegiance to ISIS.”

According to CBS News, ISIS has now taken credit for the shooting, saying "the attack was “carried out by an Islamic State fighter."

It's also come out that Mateen was investigated by the FBI in 2013 and 2014. It's unclear what triggered the probe, but the cases were closed. "He’s a known quantity," a source told Daily Beast. "He’s been on the radar before."

"He was subject to an FBI investigation, there was some information passed along that caused them to look at him, but it didn’t seem to rise to the level of terrorism," Rep. Pete King, Chairman of the House Committee on Homeland Security, told the Daily News.

Mateen, who was born in New York, was married for a time to a woman from New Jersey. That woman, whose name hasn't been released, told the Washington Post that she moved to Florida to marry him after they met eight years ago. She said he was abusive during their marriage, which lasted from April 2009 to July 2011. "He was not a stable person,” said the ex-wife. "He beat me. He would just come home and start beating me up because the laundry wasn’t finished or something like that."

The ex-wife's father echoed her statements, telling the Daily News, "After three or four months she was abused by him and then we went and picked her up from there. We even called cops to pick up her belongings."

Mateen, who had two separate licenses to carry firearms in Florida, worked for private security company G4S. He was armed with an AR-15-style assault rifle and a handgun when he entered the nightclub, Pulse, around 2 a.m. Sunday morning, taking hostages and engaging in a protracted gun battle with cops until around 5 a.m.

One of Mateen's former coworkers spoke to Miami Herald reporter David Ovalle about his behavior and language: