ORLANDO, Fla. » The FBI is investigating reports that Orlando gunman Omar Mateen had been a regular at the gay nightclub he attacked and had used gay dating apps, a U.S. official briefed on the case said today.

A number of possible explanations and motives for the bloodbath that left 49 victims dead have emerged, with Mateen professing allegiance to the Islamic State group, his ex-wife saying he was mentally ill, and his father suggesting he was driven by hatred of gays.

The picture grew more complex as patrons of the Pulse came forward to say that they had seen the 29-year-old American-born Muslim there a number of times or that he had been using gay dating apps. Mateen had a wife and 3-year-old son.

Jim Van Horn, 71, said Mateen was a regular at the club. “He was trying to pick up people. Men,” Van Horn told The Associated Press. While acknowledging he didn’t know Mateen well, Van Horn said: “I think it’s possible that he was trying to deal with his inner demons, of trying to get rid of his anger of homosexuality.”

The official who said the FBI is looking into those reports was not authorized to discuss the investigation publicly and spoke on condition of anonymity.

Wielding an AR-15 semi-automatic rifle and a handgun, Mateen opened fire at the club early Sunday in a three-hour shooting rampage and hostage siege that ended with a SWAT team killing him. It was the deadliest mass shooting in modern U.S. history. During the attack, he called 911 to profess allegiance to the Islamic State group.

Six of the more than 50 wounded were listed in critical condition today and five others were in guarded condition, Dr. Michael Cheatham of Orlando Regional Medical Center said at a hospital news conference in which doctors recalled victims arriving in “truckloads” and “ambulance-loads” the night of the rampage.

A choked-up Dr. Chadwick Smith described calling in additional staff members and telling them, “This is not a drill, this is not a joke.” He said everyone answered: “I’ll be right there.”

President Barack Obama will visit Orlando on Thursday to pay his respects to the victims and stand in solidarity with the community, the White House said.

The Orlando Sentinel and other news organizations quoted other regulars from Pulse as saying they, too, had seen Mateen at the club repeatedly.

“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 Sentinel. He said he saw Mateen inside at least a dozen times.

Gay dating app Jack’d did not immediately return a call for comment, Grindr officials said they “will continue to cooperate with the authorities and do not comment on ongoing investigations,” and Adam4Adam spokesman David Lesage said the company is looking at conversations and profiles in the Orlando area for any activity by Mateen but hasn’t found anything yet.

Asked about reports that Mateen may have been in the club before, his father, Seddique Mateen, said from his home in Port St. Lucie, Florida, that his son may have been “scouting the place.” Asked if his son was gay, the Afghan immigrant replied: “No. No.”

He said that he was not aware of his son having any mental health problems and that he never saw any signs he had become radicalized. If he had seen anything differently, he said, “I would have called law enforcement immediately.”

The elder Mateen said that apart from the time his son got angry a few months ago over seeing two men kissing, he never saw any anti-gay behavior from him.

Over the past two years, the Islamic State has targeted gay men for death in keeping with its radical interpretation of Islam, throwing dozens of them from tall buildings in Iraq and Syria.

On Monday, the White House and the FBI said Mateen appears to be a “homegrown” extremist who had expressed support not just for the Islamic State, but for other radical groups that are its enemies.

“So far, we see no indication that this was a plot directed from outside the United States, and we see no indication that he was part of any kind of network,” FBI Director James Comey said Monday. He said Mateen was clearly “radicalized,” at least in part via the internet.

On Monday night, about a mile from Pulse in downtown Orlando, thousands gathered for a vigil to support the victims and survivors. The names of the dead were read aloud. It was held on the lawn of Orlando’s main performing arts venue, where mourners created a memorial of flowers, candles and notes for the victims.

Many said they felt compelled to attend because of the role Pulse played in their lives.

“It was a place that a young 20-year-old who wasn’t openly gay felt safe for the first time,” said Cathleen Daus, now 36, who worked at Pulse in her 20s. “Pulse gave me confidence, made me realize I was normal and so much like everyone else.”

Some, including Jason Primar, who lost two friends in the massacre, released balloons that floated high above the downtown skyscrapers.

Primar went to Pulse at 2 a.m. Sunday, hoping for a good time with friends. When gunfijre erupted, “I felt like I was over in Iraq,” he said. He called his two friends inside; they never answered. He later discovered they died.

Comey said the FBI also was trying to determine whether Mateen had recently scouted Disney World as a potential target, as reported by People.com, which cited an unidentified federal law enforcement source.

“We’re still working through that,” Comey said.

He defended the bureau’s handling of Mateen during two previous investigations in 2013 and 2014 into possible terrorist ties. As for whether there was anything the FBI should have done differently, “so far, the honest answer is, I don’t think so,” Comey said.

Mateen was added to a terror watch list in 2013 when he was investigated, but was taken off it soon after the matter was closed, according to Comey. People who are in that database are not automatically barred from buying guns. Mateen purchased his weapons in June, long after his removal from the list.

Associated Press writers Eric Tucker in Washington; Allen Breed, Mike Schneider and Tamara Lush in Orlando; and photographers Chris O’Meara in Orlando and Alan Diaz in Fort Pierce contributed to this report.