Police have confirmed that 50 people have been killed and 53 hospitalised in the worst mass shooting in US history after a suspected terrorist opened fire at a gay nightclub in the Florida city of Orlando.
Authorities told reporters on Sunday that a "state of emergency" has been declared following the attack at Pulse Orlando in the early hours of Sunday morning.
The suspect was identified as Omar Mateen of Port St Lucie, Florida, the Associated Press reports. He was killed by officers following a three-hour stand off.
The gunman's father recalled that his son got angry when he recently saw two men kissing in Miami. He said that might be related to the attack.
So-called Islamic State's Amaq news agency said on Sunday that the militant group was responsible for the shooting, Reuters reports.
"The armed attack that targeted a gay night club in the city of Orlando in the American state of Florida which left over 100 people dead or injured was carried out by an Islamic State fighter," Amaq said.
A total of 53 people were hospitalised following the attack. A surgeon at Orlando Regional Medical Center said the death toll was likely to climb.
Mayor Buddy Dyer said all of the dead were killed with the assault rifle.
"There's blood everywhere," Dyer said.
The gunman exchanged gunfire with an officer working at the gay club known as Pulse Orlando around 2am when more than 300 people were inside.
The shooter then went back inside and took hostages, Police Chief John Mina said.
Officials confirmed the shooter had been killed by police after 5am. He had been carrying a "suspicious device".
A bomb squad and the police's hazardous material team were both in attendance at the scene in the early hours of Sunday morning.
Authorities were looking into whether the attack was an act of domestic or international terror, and if the shooter acted alone, according to Danny Banks, an agent with the Florida Department of Law Enforcement.
FBI agent Ron Hopper said there was no further threat to Orlando or the surrounding area.
When asked if the gunman had a connection to radical Islamic terrorism, Hopper said authorities had "suggestions that individual has leanings towards that."
The FBI revealed they believed it was an act of terrorism but said it was not clear whether it was domestic or international.
Authorities said they had secured the suspect's vehicle, a van, outside the club.
Relatives and friends, many in tears, gathered outside the hospital to learn the fate of loved ones.
Jackie Smith, who was inside the club, said two friends next to her were shot.
"Some guy walked in and started shooting everybody. He had an automatic rifle, so nobody stood a chance," Smith said. "I just tried to get out of there."
Mina Justice was outside the club early on Sunday trying to contact her 30-year-old son Eddie, who texted her when the shooting happened and asked her to call police.
He told her he ran into a bathroom with other club patrons to hide. He then texted her: "He's coming."
"The next text said: 'He has us, and he's in here with us,'" she said. "That was the last conversation."
Orlando police chief John Mina described how his force entered into a shoot-out with the attacker, resulting in one of his own officers getting shot.
The officer's life was saved by his helmet.
A post on the nightclub's Facebook page immediately after the event read: "Everyone get out of pulse and keep running."
Witness Ricardo Negroni, who was at the club when the shooting started, said the attacker had aimed his gun at the ceiling, which sent glass crashing to the floor.
He told Sky News: “At around 2am someone started shooting. People just dropped on the floor.
“I guess the shooter was shooting at the ceiling because you could see all the glass from the lamps falling."
The incident follows the fatal shooting on Friday of 22-year-old singer Christina Grimmie, who was killed after her concert in Orlando by a 27-year-old Florida man who later killed himself.
Grimmie was a YouTube sensation and former contestant on "The Voice."