The Orlando nightclub shooting puts the US anti-terror strategy under new scrutiny again, since the gunman who took away 50 lives in a hate-fueled rampage had been previously cleared of jihadist ties.
As the worst mass shooting in modern US history erupted on Sunday, Orlando police blasted their way into the Pulse nightspot and shot the attacker dead.
The murderous assault triggered grief but also defiance in the gay and lesbian community, and more than 100,000 marched in a planned Los Angeles Gay Pride parade.
In New York, the Tony Awards for musical theatre went ahead as planned but were dedicated to the victims of the massacre.
“We know enough to say that this was an act of terror and an act of hate,” President Barack Obama said, as the FBI investigated the shooter.
Prominent US Muslim figures, Pope Francis and world leaders condemned the attack, which is being treated as the worst act of terror on US soil since September 11, 2001.
The FBI admitted that 29-year-old Omar Mateen had previously been investigated -- but cleared -- for ties to a US suicide bomber.
Special agent Ronald Hopper also said Mateen was reported to have made a 911 call pledging allegiance to IS shortly before the massacre.
Meanwhile, the IS-linked news agency Amaq said without providing evidence that one of its fighters carried out the attack.
But the group, which has previously been a conduit for Islamic State statements cited only “a source,” leading experts to doubt the claim.
Terrified survivors described how the gunman raked club-goers with bullets, prompting a police SWAT team to storm the venue.
Mateen was born to Afghan parents in New York in 1986 and lived in Port St Lucie, Florida, about a two hour drive from Orlando.
His father Mir Seddique told NBC News his son may have been motivated by homophobia, insisting: “This had nothing to do with religion.”
The suspect’s ex-wife, who divorced him in 2011, told reporters he had been violently abusive to her but was not especially religious.
But the FBI’s Hopper told reporters Mateen’s behavior had raised red flags well before Sunday’s attack.
In 2013, he was probed by the bureau after making inflammatory comments to co-workers that suggested terrorist ties.
Later, in 2014, he was again questioned by agents investigating his contacts with Moner Mohammad Abusalha, a fellow Floridian.
Abusalha became notorious as the first US citizen to carry out a suicide bombing in Syria, and was reportedly a member of an al Qaeda affiliate.
“We determined the contact was minimal and did not constitute a substantive relationship or a threat at that time,” Hopper said.
The Orlando atrocity came at the height of what is already a heated US presidential election campaign, and the main candidates were quick to react.
Democratic flag-bearer Hillary Clinton postponed a joint campaign rally with Obama and tweeted that her “thoughts are with those affected by this horrific act.”
Her Republican rival Donald Trump, meanwhile, lost no time in claiming the attack proved he was right to promise a ban on Muslims entering the US.
Trump demanded Obama resign for failing to publicly blame the massacre on “radical Islam,” and vowed to make a speech on security policy Monday.
“If we do not get tough and smart real fast, we are not going to have a country anymore,” Trump declared.
Events at Pulse unfolded over a three hour period from around 2.00 am (0600 GMT) when shots rang out amid the throbbing music.
Police said the gunman was armed with an assault rifle and a handgun.
A policeman working “extra duties” at the club responded and two other officers exchanged fire with the suspect.
Police stormed the venue after the suspect fell back inside, and broke through a wall with a wheeled armored vehicle known as a BearCat.
It was unclear whether all the victims were killed by the gunman or if some died in crossfire during the assault to rescue his hostages.
Witness Janiel Gonzalez described scenes of mayhem as the gunman sprayed revelers with bullets.
“It was like complete chaos,” he told AFP. “It was like a scene out of a movie. People were screaming ‘Help me, help me, I’m trapped.’“
“People were getting trampled. There was no clear exit sign at the club, so we didn’t know which door to take or where to go.
The massacre -- which coincided with Gay Pride month in the United States -- cast a long shadow over the parade in Los Angeles.
Chillingly, a man was arrested in nearby Santa Monica with weapons and explosives in his car.
Police said he had no known connection to the Orlando bloodshed.