U.S. agents were investigating the alleged Islamist links of a slain gunman Sunday after an assault on a Florida gay nightclub left 50 dead.

In what became the worst mass shooting in modern U.S. history, carnage erupted at the Pulse club in Orlando before the attacker was shot dead by a police SWAT team.

“We know enough to say that this was an act of terror and an act of hate,” President Barack Obama said.

Pope Francis and world leaders joined him in condemning the attack, which is being treated as the worst act of terror on U.S. soil since Sept. 11, 2001.

The FBI revealed that 29-year-old Omar Mateen, who died in an exchange of fire with police, had previously been investigated for ties to a U.S. suicide bomber.

Special Agent Ronald Hopper said he had been cleared by the previous probe, but was believed to have made a 911 call pledging allegiance to IS before the massacre.

In another apparent jihadist connection, the IS-linked news agency Amaq said without providing evidence that one of its fighters carried out the attack.

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 two hours’ drive from Orlando.

His father Mir Seddique told NBC News his son may have been motivated by an aversion to homosexuals, 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’s behavior 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 coworkers that suggested terrorist ties.

“Ultimately we were unable to verify the substance of his comments and the investigation was closed,” Hopper said.

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 U.S. citizen to carry out a suicide bombing in Syria, and was reportedly a member of an al-Qaida 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 U.S. 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 demand a ban on Muslims entering the U.S.

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, in a campaign statement.

Events at Pulse unfolded over a three-hour period from around 2 a.m. when shots rang out amid the throbbing music.

Police said the gunman 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.

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 police assault.

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 a parade Sunday in Los Angeles.

Chillingly, a man was arrested in nearby Santa Monica with weapons and explosives in his car, saying he wanted to “harm” the Los Angeles event. Police said he had no known connection to the carnage in Orlando. (AFP)