Omar Mateen was a regular at the gay nightclub where he murdered 49 people and injured 53 more, witnesses have claimed.

Mateen, who was armed with an assault rifle and handgun, entered Pulse nightclub in Orlando and opened fire, killing and injuring dozens before a SWAT team knocked down a wall of the club and shot him.

But that wasn't the first time he'd been there, the Los Angeles Times and the Orlando Sentinel report.

One regular at the club said he had seen Mateen, whose father claimed he was very homophobic, at least a dozen times there before the massacre.

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"Sometimes he would go over in the corner and sit and drink by himself,' Ty Smith, who also uses the name Aries, told the Sentinel.

"Other times he would get so drunk he was loud and belligerent."

One club regular told the Los Angeles Times that he frequently messaged Mateen on a gay dating app called "Jack'd".

Remarks that Mateen drank heavily conflict with his apparently strict adherence to his Muslim faith, including regular worship at a mosque in his home town of Port St. Lucie - where he was quiet and kept to himself.

Smith said he'd seen Mateen at Pulse "at least a dozen times", joining three others who said they'd seen the shooter at the club before.

"We didn't really talk to him a lot, but I remember him saying things about his dad at times,' Smith said. "He told us he had a wife and child."

Mateen had in fact been married twice before his death. His first wife, Sitora Yusufiy, met him online in 2009 and married him soon after.

President Barack Obama says there's no clear evidence that the shooter at an Orlando nightclub was directed to conduct his attack or part of a larger plot.

But she sought divorce in 2011, after he turned violent.

"After a few months he started to beat me," she said. "He was mentally unstable... he was obviously disturbed. I know he had a history of [taking] steroids."

He then married Noor Zahi Salman, with whom he had a son. The pair got a mortgage together in 2013, but they were living at separate addresses when he went on his rampage.

Mateen's father, Seddique Mir Mateen, denied that the attack had anything to do with religion, and said his son had become enraged after seeing two men kissing in Miami months before.

Classmate Samuel King said Mateen was not homophobic when he knew him in the year after their 2004 graduation.

"What is shocking to me is that the majority of the staff at Ruby Tuesday's when I worked there were gay," he said. "He clearly was not anti-(gay), at least not back then. He did not show any hatred to any of us. He treated us all like the individuals we were. He always smiled and said hello."

King, who characterized Mateen as friendly and talkative, said: "Something must have changed."