The Islamic State group claimed slain shooter Omar Mateen was acting as "one of the soldiers of the caliphate in America" when he attacked the Pulse club in the Florida resort city, an assault that ended when police stormed the venue.
But investigators are probing whether he was a jihadist on a mission or a vicious "lone wolf" inspired by Islamist propaganda to carry out what President Barack Obama dubbed "an act of terror and an act of hate."
Forty-nine people were killed and 53 others wounded in the attack, which was also the deadliest mass shooting in US history.
FBI chief James Comey said: "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."
But he also said the FBI was "highly confident" Mateen had been "radicalized" while consuming online propaganda and that he had claimed to be working for Islamic State leader Abu Bakr al-Baghdadi in a series of calls during the attack.
"And of course, we are working to understand what role anti-gay bigotry may have played in motivating this attack," he said, stressing that the investigation was at an early stage.
President Barack Obama said Monday there was no evidence that the massacre of 49 people in a Florida nightclub was directed from abroad or was part of a larger plot.
A couple hug and kiss in front of a makeshift memorial to remember the victims of a mass shooting in Orlando, Fla., in New York, Sunday, June 12, 2016. Source: AAP
After being briefed on America's most deadly mass shooting in modern times by top security aides, Obama said "we don't yet know" the shooter's motivations.
Obama said Mateen did appear to have been "inspired by various extremist information that was disseminated over the Internet."
"At this stage we see no clear evidence that he was directed externally," Obama said.
"It does appear that at the last minute he announced allegiance to ISIL, but there is no evidence so far that he was in fact directed by them," he said, using an acronym for the Islamic State group.
"There's also no direct evidence that he was part of a larger plot."
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The slaughter triggered worldwide shock and outrage, but has also raised questions about US counter-terror strategy and gun laws. The suspect was allowed to legally buy a rifle and handgun despite raising red flags.
"There are certain common sense things that Congress could do that would make it harder for any individual to get their hands on a weapon of war," White House spokesman Josh Earnest said.
The FBI admitted it had previously investigated the 29-year-old American of Afghan descent, but had cleared him of extremist ties.
Relatives and acquaintances have painted a picture of a violent and unstable young man who had beat his ex-wife and expressed homophobic views.
Source: AFP
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Gay pride month
Police have identified 48 of the 49 victims -- mainly clubgoers celebrating Latin Night at one of Orlando's most prominent gay venues at the start of Gay Pride month. Many of the wounded were still in hospital.
But city mayor Buddy Dyer said many more were saved when police crashed an armored car through a wall after the gunman barricaded himself in a bathroom with hostages.
As the club was closing at around 2:00 am, Mateen began a gun battle with an off-duty officer working at the club. More officers responded, and the gunman fell back to the bathroom, where he seized hostages.
"At that time, we were able to save dozens and dozens of people," Orlando police chief John Mina told reporters.
'Complete chaos'
Witness Janiel Gonzalez described scenes of mayhem as the gunman sprayed revelers with bullets.
"It was like complete chaos," witness Janiel Gonzalez told AFP. "People were screaming 'Help me, help me, I'm trapped!'"
Mina said he later made the difficult decision to burst into the club after Mateen made comments in telephone calls about "bomb vests , about explosives."
"We believed that further loss of life was imminent," Mina told reporters at a morning press conference.
"We knew that was the right thing to do and believe we prevented a future loss of life and saved many, many lives."
Police used explosives and a BearCat armored car to punch a hole in the wall. Dozens more survivors surged out of the breach, but Mateen opened fire and was cut down by police.
Source: AAP
'Not a war zone'
Officials in Orlando, a resort city famous worldwide as the home of Walt Disney World and other amusement parks, were stunned by the tragedy but vowed that the community would pull together.
"This is not a war zone that we are living in. This is a civilized society and we had to deal with something unthinkable," Orange County Sheriff Jerry Demings said.
The attacks triggered solidarity vigils by gay and lesbian groups -- and their supporters -- across the country and abroad. More than 100,000 marched in Los Angeles at an already planned pride parade.
In New York, the Tony Awards for musical theater went ahead as planned but were dedicated to the victims of the massacre.
A woman cries and holds flowers in front of a makeshift memorial to remember the victims of a mass shooting in Orlando, Fla., in New York, Sunday, June 12 Source: AAP
The Empire State building went dark, while the spire of One World Trade Center was lit in the rainbow colors of the gay pride flag.
But no mood of national outrage could survive long in the crucible of America's 2016 presidential race, and presumptive Republican presidential nominee Donald Trump seized upon it to launch an attack on Obama and his Democratic rival Hillary Clinton.
"I think you're going to see tremendous pressure now on these weak politicians that don't know what they're doing, like our president," Trump told Fox News.
"The man has no clue. He doesn't know what he's doing and Hillary Clinton is going to be worse than him," he declared, arguing that Obama had failed to confront "radical Islam" because of "political correctness."
Clinton pushed back against the charge that she is unwilling to identify Islamist extremism as a threat, telling MSNBC: "We have to defeat radical jihadist terrorism or radical Islamism, whatever you call it. It's the same.
"But we cannot demonize, demagogue and declare war on an entire religion... it plays into the hands of ISIS and other jihadist terrorists. And so that's why I have been focused on how we tackle this issue of self-radicalization."
