Omar Mateen killed 50 people and left 53 others injured in gay nightclub massacre
The gunman suspected of carrying out America's deadliest terrorist attack since 9/11 was known to law enforcement agencies, the FBI has confirmed.
Omar Mateen, 29, was armed with a device thought to be a bomb, an assault rifle and a handgun, when he opened fire in the nightclub Pulse on Sunday morning before taking hostages.
He killed 50 people and left 53 others injured, many critically.
The attack unfolded just after 2am when an officer working as a security guard at the club raised the alarm and engaged in a gun battle with Mateen.
The gunman then took a number of hostages inside the popular gay club.
Some clubbers mistook the sound of gunshots for music and the bar posted on its Facebook account: "Everyone get out of pulse and keep running."
Police launched a mission at 5am to rescue the hostages and 11 officers stormed the club, shooting dead Mateen, who had worked as a security guard for G4S.
A statement carried by Amaq news agency, which is tied to IS, said the mass shooting was "carried out by an Islamic State fighter".
The Islamic State claim came after it emerged that the suspect reportedly swore allegiance to IS in a 911 call he made just before the attack.
The FBI confirmed the call, saying it was now "federal evidence" but officials familiar with the investigation say no evidence has yet been found of ties between Mateen, who is an American of Afghan descent, and terror groups.
Mateen, of Port St Lucie, Florida, had been interviewed in 2013 following inflammatory comments made to co-workers and again over ties to an American suicide bomber in 2014.
He was not under surveillance at the time of the attack.
According to one FBI official, he had legally purchased two firearms within the last week.
President Barack Obama has condemned the massacre as an "act of terror and an act of hate", lamenting "how easy it is" for criminals to get their hands on weapons.
The names of the first victims to be identified have now been released by Orlando authorities.
They are Edward Sotomayor Jr, 34, Stanley Almodovar III, 23, Luis Omar Ocasio-Capo, 20, Juan Ramon Guerrero, 22, Eric Ivan Ortiz-Rivera, 36, Peter O Gonzalez-Cruz, 22, Luis S Vielma, 22 and Kimberly Morris, 37.
Orlando Mayor Buddy Dyer said: "We're dealing with something that we never imagined and is unimaginable. There's blood everywhere."
Mr Grayson told Sky News authorities are also investigating whether the attack was a "hate crime perpetrated against gays" after the gunman's father said the attack had "nothing to do with religion."
Mateen's father Seddique Mir Mateen told US media the massacre, which took place as the US celebrates LGBT Pride Month, may have been triggered by the sight of a gay couple kissing in Miami.
"He saw two men kissing each other in front of his wife and kid and he got very angry," he told NBC News.