The gunman whose attack on a gay nightclub left 49 victims dead appears to have been a "homegrown extremist" who espoused support for a jumble of often-conflicting Islamic radical groups, the White House and the FBI said Monday.

As Orlando mourned its dead with flowers, candles and vigils, counterterrorism investigators dug into the background of 29-year-old Omar Mateen, the American-born Muslim who carried out the deadliest mass shooting in modern U.S. history.

"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," said FBI Director James Comey. But he said Mateen was clearly "radicalized," at least in part via the internet.

Comey said the bureau is also 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.

The FBI chief defended the bureau's handling of Mateen during two previous investigations into his apparent terrorist sympathies. 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.

Despite Mateen's pledge of fealty to the Islamic State, a murky combination of other possible motives and explanations emerged, with his ex-wife saying he suffered from mental illness and his Afghan-immigrant father suggesting he may have acted out of anti-gay hatred. He said his son got angry recently about seeing two men kiss.

The Orlando Sentinel and other news organizations quoted regular customers at the gay bar as saying they had seen Mateen there a number of times.

"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," said Ty Smith. Smith said he saw the killer inside at least a dozen times.

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

At the White House, President Barack Obama said there is no clear evidence so far that Mateen was directed by the group, calling the attack an apparent example of "homegrown extremism." Obama is traveling to Orlando on Thursday to pay his respects to the victims and stand in solidarity with the community, the White House said Monday evening.

More details of the bloodbath emerged, with Orlando Police Chief John Mina saying Mateen was "cool and calm" during phone calls with police negotiators. But the chief said he decided to send the SWAT team in and bash through a wall after Mateen holed up with hostages in a bathroom and began to talk about bombs and an explosive vest.

"We knew there would be an imminent loss of life," Mina said. As it turned out, Mateen had no explosives with him.

Five of the wounded were reported in grave condition, meaning the death toll could rise. A call went out for blood donations.

In Orlando, mourners piled bouquets around a makeshift memorial, and people broke down in tears and held their hands to their faces while passing through the growing collection of flowers, candles and signs about a mile from the site of the massacre. Later Monday evening, thousands gathered near the site for a vigil held on the lawn of the Dr. Phillips Center, the area's main performing arts venue. Many in the crowd said they were inspired to attend because Pulse played a huge role in their lives as gays and lesbians.

"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.

About 300 employees of the Red Lobster restaurant chain -- some in business suits, some in chef's uniforms -- emerged from the company's corporate headquarters and walked two-by-two across the street to the memorial, each carrying a red or white carnation.

"We will not be defined by the act of a cowardly hater," vowed Mayor Buddy Dyer, whose city of a quarter-million people is known around the globe as the home of Walt Disney World and other theme parks.

The tragedy hit the city's gay and Hispanic communities especially hard. It was Latino Night at the club when the attack occurred.

"As the names come out, they are overwhelmingly Latino and Hispanic names," said Christina Hernandez, a Hispanic activist. "These were not just victims of the LBGT community, but of the Hispanic community, as well. This was senseless bloodshed."

Mateen's grasp of the differences between Islamic extremist groups appeared shaky.

During three calls with 911 dispatchers, Mateen not only professed allegiance to ISIS but also expressed solidarity with a suicide bomber from the Syrian rebel group Nusra Front, and a few years ago he claimed connections to Hezbollah, too -- both ISIS enemies, according to Comey.

The FBI became aware of Mateen in 2013 when co-workers reported that the private security guard claimed to have family connections to al-Qaida and to be a member of Hezbollah, too, Comey said. He was also quoted as saying he hoped that law enforcement would raid his apartment and assault his wife and child so that he could martyr himself.

The FBI launched a 10-month preliminary investigation, following Mateen, reviewing his communications and questioning him, the FBI chief said. Mateen claimed he made the remarks in anger because co-workers were teasing him and discriminating against him as a Muslim, and the FBI eventually closed the case, Comey said.

His name surfaced again as part of another investigation into the Nusra Front bomber. The FBI found Mateen and the man had attended the same mosque and knew each other casually, but the investigation turned up "no ties of any consequence," 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, and in any case Mateen purchased his weapons in June, long after he was removed from the list.

On Sunday, the bloodshed started after Mateen approached the club around 2 a.m., exchanged fire with an off-duty officer working security, and then went inside and started gunning people down, police said.

After two other officers arrived and exchanged gunfire with Mateen, the gunman holed up in a restroom with about five club-goers. An additional 15 to 20 were in another nearby bathroom, authorities said.

Hostage negotiators began talking to Mateen.

After Mateen began to talk about explosives, Mina made the decision around 5 a.m. to blow open a wall to the bathroom. The explosives didn't penetrate the wall completely, so an armored vehicle was used to punch a 2-by-3-foot hole. Dozens of people escaped, and Mateen was gunned down as he emerged through the hole, police said.

The Islamic State's radio hailed the attack and called Mateen "one of the soldiers of the caliphate in America." But it gave no indication the group planned or knew of the attack beforehand.

Counterterrorism experts have been warning in the past few years about the danger of so-called lone wolf attackers who act in sympathy with extremist groups like the Islamic State but are not directed by them.

Mateen's father, Seddique Mir Mateen, told reporters that the massacre was "the act of a terrorist," and added: "I apologize for what my son did. I am as sad and mad as you guys are."

He wouldn't go into details about any religious or political views his son held, saying he didn't know. Asked whether he missed his son, he said: "I don't miss anything about him. What he did was against humanity."

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