Federal investigators are examining possible motives for the massacre of 49 people in a Florida nightclub - the worst mass shooting in modern US history.
The White House and the FBI said 29-year-old gunman Omar Mateen, an American-born Muslim, appeared to be a "homegrown extremist" who had touted support not just for Islamic State (IS) but 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. He said Mateen was clearly "radicalised", at least in part via the internet.
Despite Mateen's pledge of support for IS, other possible explanations for the attack have emerged. His ex-wife said he suffered from mental illness, and his Afghan-immigrant father suggested he may have acted out of anti-gay hatred. He said his son recently became angry about seeing two men kiss.
Meanwhile, thousands gathered in Orlando on Monday night for a vigil to support the victims and survivors of the Pulse nightclub shooting. It was held on the lawn of Orlando's main performing arts venue, where mourners created a makeshift memorial of flowers, candles and notes for the victims, and the names of the dead were read out.
Many in the crowd said they felt compelled to attend the vigil because of the important 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 had worked at Pulse in her 20s. "Pulse gave me confidence, made me realise I was normal and so much like everyone else."
Some, like Jason Primar, who lost two friends in the massacre, released balloons which floated high above the city's skyscrapers.
Mr Primar went to the club at 2am on Sunday, hoping to have a good time with friends. Instead, he was greeted with gunshots and became anxious about friends inside.
"I felt like I was over in Iraq," he said.
He called his two friends inside and they never answered. He later discovered they had died.
Mr Comey said the FBI was 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," Mr 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", Mr Comey said.
The Orlando Sentinel and other news organisations 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, who added that he saw the killer inside at least a dozen times.
Jim Van Horn, 71, said he was a frequent patron at Pulse and said another "regular" there was Mateen.
"He was trying to pick up people - men," he told the Associated Press outside Parliament House, another gay club.
Mr Van Horn said he met Mateen once, and the younger man told him about his ex-wife.
"My friends came out from the back and said 'Let's go take pictures on the patio'," Mr Van Horn said. "So I left. And then they told me they didn't want me talking to him, because they thought he was a strange person."
Early on Sunday, Mateen, wielding an AR-15 semi-automatic rifle and a handgun, opened fire at Pulse in a three-hour shooting rampage and hostage siege which ended with a Swat team killing him. During the attack, he called 911 to profess allegiance to Islamic State group.
At the White House, President Barack Obama said there was no clear evidence so far that Mateen was directed by the group, calling the attack an apparent example of "homegrown extremism".
He will travel to Orlando on Thursday to pay respect to the victims and stand in solidarity with the community, the White House said.
Meanwhile, 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 Mr Mina said he decided to send the Swat team in and smash through a wall after Mateen holed up with hostages in a toilet and began to talk about bombs and an explosive vest.
"We knew there would be an imminent loss of life," Mr Mina said. As it turned out, Mateen had no explosives with him.
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.
"We will not be defined by the act of a cowardly hater," vowed Mayor Buddy Dyer, whose city of a quarter of a 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.
"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."
Five of the wounded were reported to be in a critical condition, meaning the death toll could rise, and a call has gone out for blood donations.
Mateen's grasp of the differences between Islamic extremist groups appeared shaky.
During three calls with 911 dispatchers, Mateen not only professed allegiance to IS but also expressed solidarity with a suicide bomber from the Syrian rebel group Nusra Front, and a few years ago he also claimed connections to Hezbollah - both IS enemies, according to Mr 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, Mr Comey said. He was also quoted as saying he hoped 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, Mr Comey said.
Mateen's 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", Mr 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 the FBI chief.
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.
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.
Counter-terrorism 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 IS 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 apologise for what my son did. I am as sad and mad as you guys are."
He would not go into detail about any religious or political views his son held, saying he did not know. Asked whether he missed his son, he said: "I don't miss anything about him. What he did was against humanity."