Omar Mateen, who the Florida Police suspect was behind the attack on a nightclub in Orlando, always spoke about killing people and, was racist, belligerent and "toxic," a former colleague of his told media. At least 50 people were killed and another 53 injured in the shooting, said to be the worst in the history of the United States.

"He talked about killing people all the time. He was just agitated about everything, always shaken, always agitated, always mad," the New York Times reported, quoting former co-worker Daniel Gilroy as saying. Gilroy was citing instances from the time when they both were working for G4S Secure Solutions as security guards.

Talking to NBC News, Gilory said Omar was "scary in a concerning way" and "it was all the time... The things that would set him off were always women, race or religion." Omar "always referred to every other race, religion, gender in a derogatory way," Gilroy added.

According to him, Omar was also disrespectful towards women. "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," said Gilroy.

The "inflammatory" opinions he expressed to his co-workers and claims of having "terrorist ties" attracted the attention of the Federal Bureau of Investigation (FBI) in 2013, Al Jazeera quoted FBI Orlando chief Ron Hopper as saying.

No action was taken against Omar then as the FBI was "unable to verify the substance of his comments, and the investigation was closed," Hopper said. However, they interrogated him again in 2014 after finding that he may have "possible ties" to American suicide bomber Moner Mohammad Abusalha. The FBI had to close the case after they found Omar's ties with Abusalha "was minimal and did not constitute a substantive relationship or threat at that time."

Omar was homophobic

Omar's father Seddique Mir Mateen told NBC News his son was against homosexuality and a recent incident of intimacy between same-sex people had left him furious. He added that his purported crime — the attack on the gay nightclub — was not about religion.

"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 on 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."

Seddique said he was sorry for what happened in Orlando and that he was not "aware of any action he (Omar) is taking. We are in shock like the whole country."

Omar was mentally unstable; he beat me: Ex-wife

Omar was mentally "unstable" and "obviously disturbed, deeply, and traumatised," his former wife Sitora Yusifiy told the Guardian. Omar, 29, had kept Yusifiy hostage and her family had to "rescue" her from his house four months after their marriage in 2009.

"In the beginning he was a normal being that cared about family, loved to joke, loved to have fun. A few months after we were married I saw his instability, I saw his bipolar, and he would get mad out of nowhere, and that's when I started worrying about my safety," she said.

"Then after a few months he started abusing me physically, very often, and not allowing me to speak to my family, and keeping me hostage from them," she said, adding that her family came to rescue her one day "and had to pull me out of his arms."

The 29-year-old was born in New York to parents who were originally from Afghanistan.