Sally Heyman, a Miami-Dade County Commissioner, said officials have been unable to make contact with 51 individuals who "supposedly" live in the building, 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 later told reporters that 99 people were unaccounted for and that 53 others whose whereabouts were initially unknown have since been located, though he did not make clear whether everyone in the second group was alive.
"Fire and rescue are in there with their search team, with their dogs. It's a very dangerous site right now. Very unstable," Ramirez told reporters. "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."
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, a seaside enclave of 5,700 residents on a barrier island across Biscayne Bay from the city of Miami, including two who were pulled from the rubble as response teams used trained dogs and drones in the search for survivors.
Officials said the building, built in 1981, was going through a recertification process requiring repairs and that another building was being built next door, although the cause of the collapse remained unclear.
"We have 51 people that were assumed to have been there, but you don't know between vacations or anything else, so we're still waiting," Heyman told CNN by phone earlier in the day. "The hope is still there, but it's waning."
Jenny Urgelles woke up to the news that her parents' building had collapsed. She called them, but both their phones went straight to voicemail, she told WSVN 7 News.
"I am holding on to hope. I very desperate to know what's happening," she told the local Fox affiliate.
Footage from WPLG Local 10, a Miami TV station, showed a rescue team pulling a boy from piles of debris and rebar, and firefighters using ladder trucks to rescue residents trapped on balconies.
The search effort was slowed by at least one fire that burned at the site as emergency crews doused the rubble with water, local media reported.
Emergency responders and officials were still looking for people who might be in the rubble, as well as trying to identify residents who were not home at 1:30 a.m. (0530 GMT), when an entire side of the building gave way and fell to the ground below.
"We all woke up in the early morning hours to a tragic scene," said Florida Governor Ron DeSantis, who toured the scene of the catastrophe on Thursday afternoon. "We still have hope to identify additional survivors."
At least six Paraguayan nationals were among those unaccounted for, Paraguay's Ministry of Foreign Affairs said in a statement, including the sister of Paraguay's first lady, the sister's husband Luis Pettengill and their three children.
SUBJECT TO VARIOUS INSPECTIONS
The Champlain Towers South had more than 130 units, about 80 of which were occupied. It had been subject to various inspections recently due to the recertification process and the adjacent construction of a building called 88 Park, Surfside Commissioner Charles Kesl told Local 10.
Surfside Mayor Charles Burkett said construction work was being done on the roof of the Champlain Towers South but there was no indication that it caused the collapse.
"It's hard to imagine how this could have happened," Burkett told reporters. "Buildings just don't fall down."
Burkett said that part of the building with balconies facing the beach collapsed, with one floor appearing to have fallen atop another, 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," Cohen, a former vice mayor of Surfside and a resident of the building, told reporters 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."
Cohen also said there had been construction for more than a month on the building's roof.
The Miami-Dade Police have assumed control of the investigation. More than 80 fire and rescue units responded, the Miami-Dade County Fire Rescue Department wrote in a Twitter message early Thursday.
