US authorities said the attacker phoned emergency number 911 to pledge alliegance to terrorist group Islamic State and also mentioned the Boston Marathon bombers in his calls.
The gunman was killed after he crawled out through a hole in the building of the Orlando gay nightclub Pulse made by an armoured police vehicle trying to rescue hostages, and engaged in a gun battle with armed officers.
Chilling details of the attack came as it emerged 48 of the 49 victims of the incident had last night been formally identified. Five of the 53 being treated in hospital are “critical”.
Orlando police chief John Mina said negotiators had tried to reason with Mateen, 29, during the phone calls he made from the club’s toilet, where he had retreated with a group of hostages.
“He was cool and calm when he was making those phone calls to us,” Mr Mina said. “We had a team of crisis negotiators that did talk to the suspect just trying to get as much information as possible. He really wasn’t asking for a whole lot. We were doing most of the asking.”
After a three-hour stand-off, the suspect eventually emerged from the club and was shot dead by armed police. Officers had initially held off from entering the club because Mateen, a US citizen of Afghan descent, had suggested that he was wearing an explosive vest.
“The suspect came out of that hole himself… and engaged in a gun battle with officers, where he was ultimately killed,” Mr Mina said.
Police had earlier attempted to blow a hole in the side of the building, but when that failed, a decision was made to ram the venue with an armoured car. After the hole was created “dozens and dozens” of people reportedly emerged, followed by Mateen, who was still shooting, carrying a semi-automatic rifle.
Thirty-nine of the dead were killed at the club, and the others died at hospitals, Orlando mayor Buddy Dyer said. Workers removed the bodies four at a time on stretchers and loaded them into white vans.
Last night President Barack Obama said there was no clear evidence the gunman was directed by IS.
US authorities said he pledged allegiance to IS shortly before the attack but the extent of his links to the group remains unclear.
Mr Obama added: “This is an example of the kind of home-grown extremism all of us have been concerned about for a very long time.”
The attack – the deadliest mass shooting in US history – has reignited the country’s gun debate, with Mr Obama saying it was “a further reminder of how easy it is for someone to get their hands on a weapon that lets them shoot people in a school, in a house of worship, a movie theatre or in a nightclub”.
The UK government yesterday held a minute’s silence in Westminster in memory of the victims of the attacks.
Home Secretary Theresa May branded the attacks “evil” and said British police are reviewing security plans for large-scale and public events.