US authorities on Monday were investigating whether a gunman who killed 49 people at a gay nightclub in Orlando and declared his allegiance to Islamic State militants had received any help in carrying out the massacre.

The FBI and other agencies were looking at evidence inside and in the closed-off streets around the Pulse nightclub, where New York-born Omar Mateen perpetrated the deadliest mass shooting in US history, and the worst attack on US soil since September 11, 2001.

Mateen, the 29-year-old son of Afghan immigrants, was shot and killed by police who stormed the club early Sunday morning with armored cars after a three-hour siege.

Law enforcement officials were looking for clues as to whether anyone had worked with Mateen on the attack, said Lee Bentley, US Attorney for the Middle District of Florida.

“There is an investigation of other persons, we are working as diligently as we can on that,” Bentley said at a news conference.

“If anyone else was involved in this crime, they will be prosecuted.” Officials stressed they believed there had been no other attackers and had no evidence of a threat to the public.

Mateen's rampage began around 2 am Sunday when the club was packed with some 350 revelers. Many fled as the gunman raked the crowd with bullets from an AR-15 semiautomatic rifle and a pistol.

An initial wave of officers charged into the club and trapped Mateen in a bathroom, Orlando Police Chief John Mina told reporters. That action allowed many patrons to flee the club, though others were trapped in the restroom with Mateen, leading to the standoff.

“We were able to save and rescue dozens and dozens of people,” Mina said.

Police negotiated with Mateen for about three hours before breaking a hole in the wall, which allowed hostages to escape.

Mateen also emerged from the hole and was shot dead by officers, police said.

Officials said on Sunday the death toll was 50. On Monday they clarified that the figure included Mateen.

Some 53 people were wounded and 29 remain hospitalised at Orlando Regional Medical Center, the hospital said on Twitter.

'Son was not radicalised'

By Monday morning, all but one of those killed had been identified and about half the families of the dead had been notified, officials said.

Mateen called emergency services allegedly during the shooting and pledged allegiance to the leader of the militant Islamic State group, officials said.

Mateen's father said his son was not radicalised, but indicated the gunman had strong anti-gay feelings. His ex-wife described him as mentally unstable and violent toward her.

Islamic State reiterated on Monday a claim of responsibility. “One of the Caliphate's soldiers in America carried out a security invasion where he was able to enter a crusader gathering at a nightclub for homosexuals in Orlando,” the group said in a broadcast on its Albayan Radio.

The group's claim of responsibility does not mean it directed the attack, as it offered nothing to indicate coordination with the gunman.

Candidates debate response

President Barack Obama denounced the attack as an act of terror and hate and said on Monday that the gunman seemed to have been inspired by extremist ideas.

“It appears that the shooter was inspired by various extremist information that was disseminated over the internet,” Obama told reporters at the White House.

“It does appear that at the last minute he announced allegiance to ISIL (Islamic State), but there is no direct evidence so far that he was directed.”

The attack reignited the debate over how best to confront violent Islamist militancy, and immediately became a sharp point of disagreement in the campaign for the November presidential election.

Democratic candidate Hillary Clinton, speaking on MSNBC, said the United States should walk a fine line in bolstering security without demonising Muslims, and also called for tougher gun safety measures.

Trump, in interviews with CNN and Fox News, criticised the US Muslim population for not reporting suspicions to authorities, and reiterated his call for a temporary ban on Muslims entering the country.

Father 'would have arrested' son

Mateen was an armed guard at a gated retirement community, and had worked for the global security firm G4S for nine years. He had cleared two company background screenings, the latest in 2013, according to G4S.

Despite his 911 call expressing support for IS, US officials said on Sunday they had no conclusive evidence of any direct connection with foreign extremists.

Mateen's father, Seddique Mir Mateen, told reporters on Monday that he was angered by his son's actions.

“I am sorry,” the father said. “If I did know, 1pc, that he was committing such a crime myself, I would have arrested him myself, I would have called FBI.”

In an earlier interview with NBC news, the father described an incident in downtown Miami in which his son became angry after seeing two men kissing in front of his wife and child.

Another male relative, who asked that his name not be used, said in an interview the family was still grappling with the news.

“We are all mad at Omar,” the relative said, adding that Mateen's young son “is asking about his father all the time. We don't want to lie to him, but we also don't want to tell him the truth.”

Mateen's former wife, Sitora Yusufiy, told reporters near Boulder, Colorado, that she had been beaten by Mateen during angry outbursts in which he would “express hatred towards everything”.

Authorities said on Sunday that Mateen had been twice questioned by FBI agents in 2013 and 2014 after making comments to co-workers about supporting militant groups, but neither interview led to evidence of criminal activity.

Mateen visited Saudi Arabia in 2011 and 2012 for religious pilgrimages, a government spokesman said on Monday.

Homegrown attacks

Sunday's attack underlined the inherent difficulties of providing security at open public events.

“We are determined to continue living in an open and tolerant way even if such murderous attacks plunge us into deep mourning,” German Chancellor Angela Merkel said during a visit to China, as condolences poured in from around the world.

The attack came six months after a married couple in California killed 14 people at an office holiday party in San Bernardino. The couple, who were inspired by IS, died in a shootout with police hours after the mass shooting.