About 20 people died and 42 were injured in a shooting and hostage-taking early Sunday at a gay nightclub in Orlando, Florida, police said.
The FBI, who is assisting the investigation by Orlando police, said it was treating the attack as an act of "domestic terrorism" and that the deceased shooter "may have leanings toward" radical Islam.
The incident at the Pulse nightclub in the centre of Orlando began at 2 am (0600) when a police officer posted outside the building confronted the shooter, who then opened fire, police chief John Mina said.
The gunman - who Mina said was carrying an assault rife, a handgun and "some sort of device" on his body - then moved inside the club and took hostages.
Mina described him as "organized and well-prepared." Police said the shooter was not from the Orlando area and they were still trying to determine a motive for the killings.
SWAT teams stormed the club three hours later, at 5 am, after phone calls from some of the 15 people who said they were hiding inside one of the club's bathrooms.
The shooter then died in a gunfight with officers.
The police chief said 30 people were rescued from inside the club.
About 20 clubgoers were found dead inside the building, Mina said, but cautioned that police don't yet have an exact number for those killed as the club was still being cleared of "devices."
A loud explosion heard around 5 am was a "controlled explosion," police said.
The injured were transported to three area hospitals.
Nine officers were involved in the gun battle. One officer was injured in the eye, but not seriously, Mina said.
"Everyone get out of Pulse and keep running," the club wrote on its Facebook page after the attack began.
Anthony Torres, who was at Pulse for a Latin-themed night, told CNN that the "shots did not sound like a regular handgun, it sounded like it was shooting repetitively and it wouldn't stop."