Hundreds of rescue workers scoured through tonnes of rubble on Thursday after a 12-storey oceanfront residential building partially collapsed in southern Florida.
At least one person died and 99 were unaccounted for, officials said.
Sally Heyman, a Miami-Dade County Commissioner, said officials have been unable to make contact with 51 people who "supposedly" stay in the building.
It is home to a mix of people including families and part-time "snow bird" residents who spend the winter months in Florida.
Miami-Dade Police Director Freddy Ramirez said that 99 people were unaccounted for and that 53 whose whereabouts were initially unknown have since been found.
Mr Ramirez did not make it clear whether everyone in the second group was alive.
"Fire and rescue are in there with their search team, with their dogs," he said. "It's a very dangerous site right now, very unstable.
"They're in search-and-rescue mode and they will be in that mode for a while. They are not quitting. They're going to work through the night. They are not stopping."
Mr Ramirez said the numbers of known casualties and people missing were likely to fluctuate.
"I don't want to set false expectations," he said. "This is a very tragic situation for those families and for the community."
A fire official said 35 people were rescued from the building in Surfside, an enclave of 5,700 residents on a barrier island across Biscayne Bay from the city of Miami.
They including two who were pulled from the rubble as response teams used trained dogs and drones in a search for survivors.
Florida Governor Ron DeSantis said it was possible that more victims would be found in the rubble. He said he planned to go to the scene.
"We'll hope for the best in terms of additional recoveries but we are bracing for some bad news, just given the destruction that we’re seeing," Mr DeSantis said.
Built in 1981, the Champlain Towers South had more than 130 units, about 80 of which were occupied.
It was unclear how many people were inside at 1.30am when an entire side of the building pulled away and fell to the ground.
"It's hard to imagine how this could have happened," Surfside Mayor Charles Burkett said.
"Buildings just don't fall down."
Construction work was being done on the roof, Mr Burkett said, but it was unclear whether the project involved heavy equipment.
Footage from WPLG Local 10, a Miami TV station, showed a rescue team pulling a boy from piles of debris, and firefighters using ladder trucks to rescue residents trapped on balconies.
Assistant Fire Chief Raide Jadallah said that rescuers were moving in heavy equipment and working to support the remaining part of the building.
"We are still continuing the search and rescue operations," Mr Jadallah said.
Mr Burkett said that part of the building with balconies facing the beach had "pancaked", meaning one floor appears to have fallen on another, sending part of the structure cascading down.
"The back of the building, probably a third or more, is totally pancaked," he said.
Resident Barry Cohen and his wife were rescued from the building.
"At first it sounded like a flash of lightning or thunder," Mr Cohen, a former vice mayor of Surfside, said at the scene.
"But then it just kept on, steadily for at least 15 to 30 seconds. It just kept on going and going and going."
Mr Cohen said construction had continued for more than a month on the building's roof.