Omar Mateen orlando shooting.jpg
An undated handout photogaph made available by Orlando Police on 12 June 2016 shows suspected Orlando club shooter Omar Mateen.
Photograph: EPA/ORLANDO POLICE

The gunman who killed 50 people at a gay nightclub in an Orlando, Florida, called police during the massacre to talk about the Islamic State terrorist group, an FBI official said Sunday.

Ronald Hopper, FBI special agent in charge at the scene, said the gunman was Omar Mateen, 29, a US citizen born in New York to Afghan parents.

Mateen shot his way into the Pulse nightclub at 2 am (0600) Sunday, and died in a shootout with police who stormed building about three hours later, authorities said. The incident left 53 people wounded, many in critical condition.

After the shooting began, Mateen made telephone calls to 911 emergency services in which he made comments "general to the Islamic State" movement, Hooper said. He would not directly confirm media reports that Mateen pledged adherence to the terrorist group.

Hopper said the FBI investigated Mateen in 2013, speaking to witnesses and interviewing him twice over reported statements in support of Islamist militants, but found no conclusive evidence against him.

In 2014, the FBI probed alleged ties between Mateen and a US citizen who became an Islamic State suicide bomber, including another interview, but agents determined that "contact was minimal," Hopper said.

Mateen was "not under current investigation at the time of this incident and was not under surveillance."

The SITE Intelligence Group, which monitors jihadist communications on social media, republished a newsflash in the name of Islamic State's Aamaq news agency claiming the Orlando gunman was "an Islamic State fighter."

Investigators said Mateen legally purchased the firearms used in the shooting within the last week.

In Washington, flags at the White House were lowered to half staff in mourning for what President Barack Obama called "the most deadly shooting in American history."

He said the "horrific massacre" was being treated as terrorism, though motives were still being investigated.

"We know enough to say that this was an act of terror and an act of hate," he said.

Obama said that an attack "on any American, regardless of race, ethnicity, religion or sexual orientation, is an attack on all of us."

Orlando's mayor declared a city emergency, and Florida Governor Rick Scott declared a state of emergency for Orange County.

"This is clearly an act of terror," Scott said.

Orlando Police Chief John Mina it would take time for crime scene technicians to collect evidence and remove all the bodies from the building.

Mina said the attacker, who had an AR-15 assault rifle, a handgun and an "unknown number of rounds," acted alone, with no evidence of outside help.

The nightclub was hosting a Latin-themed night, and an off-duty police officer working security outside first confronted the gunman, who opened fire, moved inside the club and took hostages.

"Everyone get out of Pulse and keep running," the club wrote on its Facebook page after the shooting began.

SWAT teams stormed the club at 5 am, rescuing 30 people who were still inside, many hiding in a bathroom.

Local authorities said 14 officers from the Orlando police and Orange County Sheriff's Department exchanged fire with the slain gunman, and were now on routine leave under rules for police shootings.

Mateen's father told NBC News that the attack "has nothing to do with religion."

Seddique Mir Mateen told the broadcaster that his son had become angered after seeing two men kissing in Miami several months ago.

"We are apologizing for the whole incident. We weren't aware of any action he is taking. We are in shock like the whole country," he was quoted by the network as saying.

Authorities opened crisis counseling hotlines, and 1,500 people queued at a blood bank in response to an appeal for donations.

The massacre was carried out on a weekend when annual gay pride rallies were being held in Washington, Los Angeles and other cities.

Police in Santa Monica, California, arrested a "heavily armed" man who said he had planned to go to a gay pride rally Sunday in Los Angeles, Mayor Eric Garcetti said.

The Los Angeles Times reported that police investigating a report of a prowler found a man who had a car with license plates from Indiana, and which contained assault rifles, ammunition and suspected explosives.

The incident was "completely unrelated" to the Orlando shooting, the mayor said.

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