Fifty people are dead after a gunman opened fire at an Orlando gay club, the city's mayor said.
In the worst mass shooting in US history a man thought to have been armed with an assault-type rifle and a handgun opened fire at the Pulse nightclub, mayor Buddy Dyer told reporters.
The gunman, who also took hostages, was later killed in a gunfight with Swat officers in the Florida city.
More than 53 people have also been taken to hospital following the incident which happened around 2am (7am BST) local time.
Describing the scene to reporters at a news conference, Dyer said there was "blood everywhere".
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.
"This is an incident, as I see it, that we certainly classify as 'domestic terror incident'," Orange County Sheriff Jerry Demings said.
The suspect was identified as Omar Mateen of Port St Lucie, Florida. Representative Alan Grayson named the shooter, citing law enforcement officials.
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".
Orlando is a popular tourist destination with British families. The Foreign and Commonwealth Office have since issued advice to UK nationals who may be in the city to "avoid the area if possible and follow the advice of local authorities".