At least 50 people have been killed and some 53 injured in a shooting at a nightclub in Orlando, Florida, the mayor Buddy Dyer says.
Orlando Police said officers entered the Pulse Club about three hours after the shooting unfolded and killed the gunman who had taken hostages.
They called it an act of terrorism, but said they did not yet know if it was domestic or international.
Relatives have been gathering at local hospitals desperate for news. Many had received calls and texts from loved ones inside the club as the siege began and some have heard nothing since, BBC reported.
Orlando Police Chief John Mina said the attack began at 02:00 (0600 GMT). There was an exchange of fire with a police officer working at the club, but it is unclear whether that was inside or outside the venue. After this a "hostage situation" unfolded, he said.
At least 20 bodies have been found in the club itself, and some 42 people have been taken to hospital to be treated for their injuries, Chief Mina said. A clubber earlier described a situation of chaos outside, as the number of casualties became apparent.
"There were just bodies everywhere," Christopher Hansen said. "In the parking lot, they were tagging them—red, yellow—so they knew who to help first and who not help first. They had to find the bullets. Just blood everywhere."
Some of the injured were reportedly brought to the Orlando Regional Medical Center in police pickups.
More than 100 people were reported to have been in the club when the attack began.
"We heard rapid fire go off. In the room I was in, people went down to the floor. I wasn't able to see the shooter or people get hurt," Ricardo Negron Almodovar told BBC News.
"At some point, there was a brief pause and a group of us got up and went to the exit that leads to the patio area outside. We found an exit and after that ... I just ran."
According to the Mass Shooting Tracker, the US last year suffered 372 mass shootings, defined as a single incident that kills or injures four or more people. Some 475 people were killed and 1,870 wounded.
The latest incident came as Orlando was still reeling from the fatal shooting on Friday night of 22-year-old singer Christina Grimmie following a concert in the city.
She had been signing autographs when she was shot by 26-year-old Kevin James Loibl who then killed himself. It is not clear what Loibl's motive was.
