Orlando gay nightclub mass shooting suspect Omar Mateen, 29 is shown in this undated photo. [Photo/Agencies]

'UNIMAGINABLE'

The shooting evolved into a hostage situation, which a team of SWAT officers ended around dawn when they used armored cars to storm the club before shooting dead the gunman. It was unclear when the victims were killed.

The number of dead shocked officials in Orlando, a city of 270,000 people and home to tourist attractions including the Disney World resort. They had initially put the death toll at 20.

"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 hospital.

Orlando Regional Medical Center hospital said it had admitted 44 victims, including nine who died, and had carried out 26 operations on victims.

The city of Orlando, which drew 62 million visitors in 2014,

began releasing names of the victims on Sunday, with the first four identified as Edward Sotomayor Jr., Stanley Almodovar, Luis Omar Ocasio-Capo and Juan Ramon Guerrero.

PRIOR FBI INTERVIEWS

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.

A dozen unmarked police cars had gathered around a Port Saint Lucie house that appeared to be linked to the gunman.

Likely Republican presidential nominee Donald 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.

Democratic candidate Hillary Clinton tweeted a brief statement after the attacks, but did not speculate on the motives of the gunman.

Florida Governor Rick Scott called for Americans to hold a moment of silence at 6 p.m. EDT (2200 GMT) to commemorate the dead. World leaders including Pope Francis, Britain's Queen Elizabeth and the leaders of Canada and Afghanistan condemned the attack.

Mateen was born in New York of parents who were immigrants from Afghanistan, according to a federal official who spoke on the condition of anonymity.

If confirmed as an act of terrorism, it would be the deadliest such attack on US soil since Sept. 11, 2001, when al Qaeda-trained hijackers crashed jetliners into New York's World Trade Center, the Pentagon and a field in Pennsylvania, killing some 3,000 people.

Mateen also referenced the ethnic Chechen brothers who killed three people in a bombing attack at the Boston Marathon in 2013, according to law enforcement officials.

The Orlando 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 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."