ORLANDO: Fifty people died on Sunday (Jun 12) when a gunman allegedly inspired by the Islamic State group opened fire inside a gay nightclub in Florida, in the worst terror attack on US soil since Sep 11, 2001.
Scenes of carnage unfolded at the Pulse club in Orlando where another 53 people were injured, some critically, in the worst mass shooting in American history.
"We know enough to say that this was an act of terror and an act of hate," President Barack Obama said, as he led Pope Francis and Western leaders in condemnation.
Police cars and fire trucks seen outside the Pulse nightclub on Sunday. (Photo: Orlando police's Twitter page)
The FBI revealed on Sunday afternoon that the 29-year-old gunman Omar Mateen, who died in an exchange of fire with police, had previously been investigated for ties to an American suicide bomber.
Special Agent Ronald Hopper told reporters Mateen had been cleared by the previous probe, but was believed to have made a 911 call pledging allegiance to IS before the massacre.
Strengthening the theory of a militant connection, the IS-linked news agency Amaq said that one of its fighters carried out the attack.
"The attack that targeted a nightclub for homosexuals in Orlando, Florida and that left more than 100 dead and wounded was carried out by an Islamic State fighter," it said, in a terse statement quoting an unidentified "source".
Terrified survivors described how the gunman raked the club with bullets, prompting a police SWAT team to storm the venue and shoot him dead.
Omar Mateen was born to Afghan parents in 1986 and lives in Port St Lucie, Florida, about two hours 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 the Washington Post he had been violently abusive to her but was not especially religious.
But the FBI's Hopper told reporters Mateen had sent up a red flag well before Sunday's attack over alleged links to Islamist extremists.
First, in 2013, he was probed by the bureau after making inflammatory comments to co-workers that suggested terrorist ties.
"Mateen was interviewed twice," Hopper said. "Ultimately we were unable to verify the substance of his comments and the investigation was closed."
Later, in 2014, he was questioned by agents investigating his contacts with Moner Mohammad Abusalha, a fellow Floridian and the first US citizen to carry out a suicide bombing in Syria.
"We determined the contact was minimal and did not constitute a substantive relationship or a threat at that time," Hopper said.
Map of Orlando, Florida, locating the gay nightclub targeted in the shooting. (Photo: AFP/Jonathan Jacobsen/Kun Tian)
'WHEN WILL WE GET TOUGH'
The Orlando atrocity comes 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 - who postponed a joint campaign rally with Obama in light of the attack - tweeted that her "thoughts are with those affected by this horrific act."
Woke up to hear the devastating news from FL. As we wait for more information, my thoughts are with those affected by this horrific act. -H— Hillary Clinton (@HillaryClinton) 12 June 2016
Her Republican rival Donald Trump meanwhile lost no time in claiming the massacre proved he has been "right on radical Islamic terrorism." "When will we get tough, smart and vigilant?" tweeted the billionaire, who has called for Muslims to be banned from travelling to the United States.
Events at Pulse unfolded over a three-hour period from at around 2.00am (2.00pm Singapore time) when shots rang out amid the throbbing music.
Police said the suspect was armed with an assault rifle and a handgun. A police officer working "extra duties" at the club responded, joined by two other officers, who exchanged fire with the suspect.
"The suspect at some point went back inside the club where more shots were fired. This did turn into a hostage situation," police chief John Mina said. "At approximately 5.00am (5.00pm Singapore time) this morning, the decision was made to rescue hostages that were in there."
Orlando police chief John Mina and other city officials answering the media's questions. (Photo: Reuters/Kevin Kolczynski)
Police then stormed the venue, using explosives and breaking through a wall with a wheeled armored vehicle known as a BearCat.
Mina said about 30 people were rescued during the operation and that the police priority now would be to identify the victims and notify next of kin. It was unclear whether all the victims were killed by the gunman or if some died in the ensuing shootout.
Witness Janiel Gonzalez described scenes of mayhem as the gunman sprayed revellers 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.
"It was just really, really, really crazy," said the young man, still missing three friends after the attack.
The massacre - which coincided with gay pride month in the United States - cast a long shadow over a parade Sunday in Los Angeles, amid reports that a man had been arrested with weapons on suspicion of wanting to target the Californian event.
"We're all still in shock," said Perry Handy, a 48-year-old attorney. "I've been coming to the parade for 20 some years and last night's event was clearly a step backward in our culture."
Friends and family members embrace outside the Orlando Police Headquarters during the investigation of a shooting. (Photo: Reuters)