The former wife and acquaintances of Orlando nightclub shooter Omar Mateen who killed 49 people suggested that he was gay as he frequented gay clubs in Florida for years.

"Sometimes he would be there for a few weeks at a time and maybe on the weekends, and then you wouldn't see him for a little while, then you'd see him again, and that's the way it kind of went," James Van Horn, a customer at Pulse, told ABC News.

Federal investigators are delving deeper into Mateen's life to know his motive behind the mass shooting.

Head of security Estella Peterkin recognised Mateen as a customer at the club.

"We are going through the killer's life, as I said, especially his electronics, to understand as much as we can about his path and whether there were anyone else involved either in directing him or in assisting him," said FBI Director James Comey.

Mateen's former classmate, who did not want to be identified, said he and the suspect together with others would go to Florida gay bars while they were under the Indian River State College training programme as early as 2006.

He said one night, Mateen asked him if he was gay and told him, "You would be my kind of guy."

Van Horn said Mateen "would try to meet people and try to bump up against people and put his arm around and maybe try to dance with them or something, because that's what everybody tries to do."

Kevin West of Florida told KTRK that Mateen contacted him on a gay dating app.

"When he first contacted me, he was asking ... what clubs are popping and things of that sort and what are good places to go," said West.

Ex-wife Sitora Yusufiy, who was married to the suspect briefly in 2009, said she noticed his emotional instability.

He would express his anger towards [a] certain culture, homosexuality, because in ... Islamic culture, it is really not tolerated, homosexuality, and I know at the time he was trying to get his life straight and follow his faith. I guess that created some confusion between that, and there [were] definitely moments that he would express his intolerance to homosexuals," she said.

When asked if she thought he was gay, she said, "At this point, I think he might have been."

"You know, knowing somebody and living with them every day... I can honestly say I had no clue. But from everything that has occurred and connecting the pieces to what I knew myself from my time and experience, I would not be surprised," she said.

The former classmate of Mateen told The Palm Beach Post that he once picked him up at a bar.

He said he believed Mateen was gay but was not open about it.

"He just wanted to fit in and no one liked him," he said. "He was always socially awkward."