Fifty people are now known to have died in Saturday’s shooting in a gay nightclub in Orlando, Florida, and a “state of emergency” has been declared, the city’s mayor Buddy Dyer has said.
The gunman, named by officials as Omar Mateen, was killed after taking hostages.
Another 53 people were injured in the shooting at the Pulse club.
It is the worst mass shooting in recent US history. Police described it as an act of terrorism.
Officials said the killings were likely to be ideologically motivated, though there was no information that the gunman was associated with a particular group.
However, US Congressman Alan Grayson said it was “no coincidence” the attack happened in a gay club.
It is believed that the suspect, who comes from the town of Port St Lucie and is of Afghan descent, 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.
One woman said she had received a text message and phone call from her daughter inside the club, who said she had been shot in the arm.
Another witness told local TV reporter Stewart Moore that his friend was still locked inside the building with a gunshot wound.
Witnesses said they heard at least 20 gunshots as the incident unfolded.
More than 100 people were reported to have been enjoying a Latin-themed night in the club, which calls itself the hottest gay bar in the city.
Ricardo Negron Almodovar told BBC News that the shooter opened fire at around 02:00 local time (06:00 GMT), as the club night was coming to a close.
“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.
“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.”
Another man, Anthony Torres, said he heard people screaming that others in the nightclub were dead.
The incident comes as Orlando is 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.