Working for the global security firm G4S during the past nine years, Mateen was an armed guard for a gated retirement community in South Florida, and had cleared two company background screenings, the latest in 2013, according to G4S.

Orlando shooter Omar Mateen was a homophobe and wife beater, reports say

Preliminary investigations suggested the attack was inspired by Islamic State militants, though there was no immediate evidence that Mateen had any actual ties to the group, law enforcement officials said.

As the shooting rampage was unfolding, Mateen “made calls to 911 this morning in which he stated his allegiance to the leader of the Islamic State,” said Ronald Hopper, the FBI’s assistant special agent in charge on the case.

Shots rang out at the crowded Pulse nightclub in the heart of Orlando, about 25km northeast of the Walt Disney World Resort, as some 350 patrons were attending a Latin music event in conjunction with gay pride week celebrations. Clubgoers described scenes of carnage and pandemonium, with one man who escaped saying he hid under a car and bandaged a wounded stranger with his shirt.

“Words cannot and will not describe the feeling of that,” Joshua McGill said in a posting on Facebook. “Being covered in blood. Trying to save a guy’s life.”

Jon Alamo had been dancing at the Pulse for hours when he wandered into the club’s main room just in time to see the gunman. “You ever seen how Marine guys hold big weapons, shooting from left to right? That’s how he was shooting at people,” he said.

“My first thought was, oh my God, I’m going to die,” Alamo said. “I was praying to God that I would live to see another day.”

Pulse patron Eddie Justice texted his mother, Mina: “Mommy I love you. In club they shooting.” About 30 minutes later, hiding in a bathroom, he texted her: “He’s coming. I’m gonna die.” As Sunday wore on, she awaited word on his fate.

Fifty-three people were wounded in the rampage. It ranked as the deadliest single U.S. mass shooting incident, eclipsing the massacre of 32 people at Virginia Tech University in 2007.

“We know enough to say this was an act of terror, an act of hate,” Obama said in a speech from the White House. “As Americans, we are united in grief, in outrage and in resolve to defend our people.”

US officials cautioned, however, they had no conclusive evidence of any direct connection with foreign extremists.

“So far as we know at this time, his first direct contact was a pledge of bayat [loyalty] he made during the massacre,” said a US counterterrorism official. “This guy appears to have been pretty screwed up without any help from anybody.”

Danny Banks, special agent in charge of the Florida Department of Law Enforcement Danny Banks, was less circumspect. “Do we consider this an act of terrorism? Absolutely, we are investigating this from all parties’ perspective as an act of terrorism,” he said.

“Whether that is domestic terrorist activity or an international one, that is something we will certainly get to the bottom of.”

The attacker was carrying an AR-15-style assault rifle and a handgun, Orange County Sheriff Jerry Demings said. He also had an unidentified “device,” said Orlando Police Chief John Mina.

The shooting was nearly certain to reignite emotional debates over American gun laws and homeland security in what is shaping up to be a vitriolic US presidential campaign between Democrat Hillary Clinton and Republican Donald Trump.

The precise sequence of events in Orlando remained murky. But authorities said the gunman burst into the club and opened fire at about 2am, then took dozens of hostages that he held at gunpoint inside a bathroom during a three-hour siege that ended when a police SWAT team stormed the building using armoured cars and killed the gunman. Police said the lives of about 30 hostages were saved.

Officials in Orlando, a city of 270,000 people, were visibly shocked at the high death toll.

“We’re dealing with something that we never imagined and is unimaginable,” Orlando Mayor Buddy Dyer said. He said 39 people died inside the club, two outside, and nine others died after being rushed to the hospital.

Video footage showed police officers and civilians carrying injured people away from the club and bending over others who were lying on the ground. Dozens of police cruisers, ambulances and other emergency vehicles could be seen in the area.

Mateen had twice been interviewed by FBI agents, in 2013 and 2014, after making comments to co-workers indicating he supported militant groups, but neither interview led to evidence of criminal activity, the FBI’s Hopper said.

Hopper said Mateen was questioned in 2014 about his contacts with Moner Mohammad Abu-Salha, a US citizen who also had lived in Florida and became a suicide bomber in Syria that year.

Sunday night, federal agents combed through Mateen’s apartment in the Atlantic coast town of Fort Pierce, about 200km southeast of Orlando, searching for clues, as numerous evidence vans sat parked outside.

“The FBI is currently in the apartment. They’ll probably be there through the night,” said St Lucie County Sheriff Ken Mascara.

Near Boulder, Colorado, Mateen’s former wife, Sitora Yusufiy, told reporters he worked for a time as a correctional officer at a juvenile detention centre in Fort Pierce, and had once sought admission to a police academy.

She said she had been beaten by Mateen during outbursts of temper in which he would “express hatred towards everything.” Eventually, she was “rescued” from Mateen by members of her family who intervened four months into a stormy marriage that ultimately ended in divorce, she said.

“I know he had a history of steroids,” Yusufiy said. She also described Mateen as “emotionally unstable,” “mentally ill” and bipolar.

Deborah Sherman, an FBI spokeswoman in Denver, confirmed that federal agents had interviewed Yusufiy in Colorado.

The imam of the Florida mosque where Mateen attended prayers for nearly 10 years described him as a soft-spoken man who would visit regularly but rarely interact with others in the congregation.

Within hours of the shooting, the presumptive presidential nominees of both major political parties weighed in with statements on the tragedy.

Trump, who has called for a temporary ban on Muslims entering the United States, said he was “right on radical Islamic terrorism” and called on Obama to resign because he did not say the words “radical Islam” in his statement responding to the shooting.

Clinton echoed Obama’s comments calling the attack both an act of terror and a hate crime, adding that the massacre “reminds us once more that weapons of war have no place on our streets.”

If confirmed as an act of terrorism, it would be the deadliest such attack on US soil since the attacks of September 11, 2001, that killed some 3,000 people.

The choice of target was especially heart-wrenching for members of the US lesbian, gay, bisexual and transgender community, said LGBT advocacy group Equality Florida.

“Gay clubs hold a significant place in LGBTQ history. They were often the only safe gathering place and this horrific act strikes directly at our sense of safety,” the group said in a statement. “We will await the details in tears of sadness and anger.”

Additional reporting by The Washington Post, Associated Press and Agence France-Presse