Afghan-origin Omar Mateen, 29, who opened fire at a gay nightclub in Orlando city of Florida, visited the club before mounting the attack and used several gay dating applications, according to witnesses.
Mateen, from Port St. Lucie in Florida, opened fire in the Pulse gay nightclub around 2.00 a.m. (local time) on Sunday and killed 49 people and wounded 53 others with his wanton shooting, making it the deadliest mass shooting in the US history and worst terror strike after the 9/11 attacks in 2001.
According to the Orlando Sentinel, at least four regulars at Pulse nightclub claim to have seen the gunman at the club "several times", Efe news reported.
"Sometimes he would go over in the corner and sit and drink by himself, and other times he would get so drunk he was loud and belligerent," Ty Smith told the Orlando Sentinel, adding that he saw Mateen inside the nightclub "at least a dozen times".
Kevin West, another club regular, said Mateen messaged him regularly for a year through the gay dating app Jack'd.
West added that an hour before the shooting, when he was dropping off a friend at the club, he saw Mateen near the club wearing a dark cap and carrying a black phone.
West later confirmed from pictures released by the Federal Bureau of Investigation after the shooting, that the person in the photos was Omar Mateen.
So far, authorities have not yet confirmed if the shooter, who was killed by an elite police squad on Sunday morning, had visited the gay nightclub before and was a user of the gay dating app.
Mateen, who was born in New York, called police before the shooting and swore allegiance to the terrorist group Islamic State, according to the police.
FBI Director James Comey revealed that Mateen was monitored for 10 months because of radicalism and was previously interviewed by the FBI, but the agency found no reasons to prohibit him to purchase weapons.
Despite the red flags Mateen raised in his interactions with others, the mass killer was able to purchase a handgun and an assault rifle legally in the days before the massacre, Trevor Velino of the Bureau of Alcohol, Tobacco, Firearms and Explosives told reporters.
He had tried to buy body armor, but the store where he tried to make the purchase doesn't sell that product, according to a store manager.
FBI Director James Comey said the agency was "highly confident" Mateen was radicalised, at least in part, by viewing extremism on the internet.
"There are strong indications of radicalisation by this killer and of potential inspiration by foreign terrorist organisations," Comey said.
The Islamic State militant group has claimed responsibility of the attack, however, the extent of its involvement is still being probed.
A report on the official online IS radio channel, Al-Bayan, described the attack as a "raid on a Crusader gathering" carried out by "one of the Caliphate's soldiers in America."
