Orlando Shooter, Omar Mateen
Undated photo from a social media account of Omar Mateen Reuters

The ex-wife of Omar Mateen, who opened fire at a gay nightclub in Orlando in the early hours of Sunday, revealed that he was "mentally unstable" and "disturbed".

The 29-year-old suspect, who used to work as a security officer, killed 50 people and wounded 53 more in the deadliest mass shooting in recent US history. Later, he was shot dead by the police.

The Islamic State Group (ISIS) claimed that they were behind the attack but the police are still not sure about the extent of involvement of this group.

The FBI said that apparently Mateen made a 911 call just before the incident to claim allegiance to ISIS.

Mateen's relatives who were interviewed by the US media said that he was not a religious person. However, they said that he had anti-gay views, mental health problems and was physically abusive to his ex-wife.

Mateen's father, Mir Seddique, who was shocked over his son's crime, said that recently his son was offended to see two gay men expressing affection on a Miami street. In a n interview with NBC News he said, "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."

He also said, "We are in shock like the whole country. This had nothing to do with religion."

Mateen, a Muslim American of Afghan descent, was very regular at the mosque. The imam of the mosque said that he came to evening prayers three or four times a week, along with his son who is about four or five years old.

Syed Shafeeq Rahman of the Islamic Center of Fort Pierce said, "He would pray and his son would play. I never expected this."

He also added the possibility that Mateen might have been radicalized on the Internet. "It must be some kind of psychological problem or anger problem," he continued.

The authorities said that Mateen used to own a small caliber gun, but he bought a handgun and a long gun just two days before the attack. The Florida Department of Agriculture and Consumer Services website showed that he had a gun license which was set to expire next year in September.

According to FBI, in 2013 and 2014, Mateen was investigated twice for possible extremist views and contact with a US suicide bomber. But he was never prosecuted.