At least 50 people have been killed and some 42 injured in a shooting at a gay nightclub in Orlando, Florida, police say.
Orlando Police said officers entered the Pulse Club about three hours after the shooting unfolded - and killed the gunman, who had taken hostages.
The BBC understands the name of the suspect was Omar Mateen, aged 29.
Police called the shooting an act of terrorism, but said they did not know if it was domestic or international.
It is believed that the suspect was not on a terrorism watch list, although he was currently being investigated for an unrelated criminal act.
Relatives have been gathering at local hospitals desperate for news of their loved ones.
Many had received calls and texts from loved ones inside the club as the siege began, and some have heard nothing since.
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President Barack Obama was briefed about the shooting and asked to receive regular updates, a White House statement said.
Orlando Police Chief John Mina said the attack began at 02:00 (06:00 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.
Chief Mina said a decision was taken at 05:00 to send a police assault team into the club after receiving texts and phone calls from some of the hostages.
"Our Swat officers exchanged gunfire with the suspect, the suspect is dead," he said.
"He appeared to be carrying an assault-type rifle and a handgun, and had some type of device on him."
An FBI spokesman said they were investigating whether the attacker was a "lone wolf". He said the attacker appeared to "have leanings towards" radical Islamist ideology but it was not yet clear whether this was a case of domestic or international terrorism.
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. Pants down, shirts cut off, they had to find the bullets. Just blood everywhere."
Some of the injured were reportedly brought to the Orlando Regional Medical Center in police pick-ups.
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