US investigators on Tuesday worked to untangle the complicated motive of the "radicalized" gunman behind the massacre at a gay club in Orlando, as reports emerged that he frequented the popular nightspot and used gay dating apps.

Omar Mateen, a 29-year-old American of Afghan descent, pledged allegiance to the leader of the Islamic State (IS) group in the midst of his killing spree, officials say -- the worst mass shooting in US history.

Forty-nine people were killed and another 53 wounded after Mateen opened fire early Sunday at the Pulse nightclub in Orlando, a resort city in central Florida mainly known as the home to Walt Disney World and other amusement parks.

Mateen was killed in a shootout with police when they stormed the venue, freeing dozens of people trapped inside for about three hours after the assault began.

The FBI said Monday it was "highly confident" that Mateen had been self-radicalized by reading extremist propaganda online.

But the investigation took a new twist after witnesses said Mateen -- a practicing Muslim with a wife and a young son -- had been a regular at Pulse.

"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, noting he had seen Mateen at the club at least a dozen times.

"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."

Another Pulse regular, Kevin West, told the Los Angeles Times that Mateen messaged him on and off for a year using a gay chat app.

Other club goers told local media and MSNBC that Mateen had been using multiple gay apps, including Grindr, with mutual acquaintances to "hook up."

And a man who attended law enforcement training classes with Mateen in 2006 told the Palm Beach Post that the slain gunman had once asked him out on a date.

"We went to a few gay bars with him, and I was not out at the time, so I declined his offer," said the former classmate, who asked not to be identified to protect his privacy.

Mateen's wife, Noor Zahi Salman, is so far not cooperating in the probe, a law enforcement official told the Sentinel.

But investigators have talked extensively with Mateen's ex-wife, Sitora Yusufiy, who said he had beat her.

Relatives and acquaintances portrayed Mateen as an unstable man prone to violence who expressed homophobic views.

- 'It should be about accepting everyone' -

Thousands of people gathered in Orlando late Monday for a vigil honoring the victims, who ranged in age from 18 to 50.

The shooting hit the city's Latino community hard -- the club was hosting a "Latin Night" when the attack occurred.

One after another, speakers urged the Florida city to unite in the face of tragedy and appealed for its lesbian, gay, bisexual and transgender community (LGBT) to refuse to yield to fear.

"It should just be about accepting everyone whether you agree with them or not," said Alex Hartdegen, a 20-year-old art student.

"It's none of your business what other people do with their lives or how they love people or who they love," she said.

Towards the end of the ceremony, as the crowd stood in silence holding up white candles, the bell of a nearby church slowly rang 49 times -- once for each of the dead.

The attacks triggered solidarity vigils across the world, including in New York, where thousands gathered at the Stonewall Inn, the birthplace of the US gay rights movement, and in Los Angeles, where pop star Lady Gaga led a tearful vigil.

"This is an attack on humanity itself... an attack on everyone," said Lady Gaga, who has often spoken out on gay rights issues.

President Barack Obama will travel to Orlando on Thursday to pay his respects to victims' families.

- 'Soldier of the caliphate' -

On Monday, FBI Director James Comey explained that agents believe Mateen had been radicalized by Islamist propaganda, but found no evidence that the shooting had been coordinated from abroad.

The Islamic State group claimed that Mateen was acting as "one of the soldiers of the caliphate in America."

US investigators believe that Mateen was a "lone wolf" inspired by Islamist propaganda to carry out what Obama dubbed "an act of terror and an act of hate."

Agents are instead looking into the possible role of anti-gay bigotry -- a possibility that gained new traction in the light of Mateen's apparent frequenting of the club.

The slaughter also raised questions about US counterterror strategy and gun laws.

The FBI said it had investigated Mateen but cleared him of extremist ties. Mateen legally bought the assault rifle and handgun used in the attack.

"We make it very easy for individuals who are troubled or disturbed or want to engage in violent acts to get very powerful weapons very quickly," Obama said.

He demanded that the Republican-controlled Congress pass legislation to curb the sale of assault-type weapons like the one used in Orlando.

- 'Help me, help me' -

Police have now identified all 49 victims. Many of the wounded remained hospitalized.

As the club was closing around 2:00 am Sunday, Mateen began a gun battle with an off-duty officer working at the club. More officers responded, and the gunman escaped to the bathroom, where he barricaded himself and seized hostages.

Witness Janiel Gonzalez described scenes of mayhem as the gunman sprayed revelers with bullets.

"It was like complete chaos," he told AFP. "People were screaming 'Help me, help me, I'm trapped!'"