By Marco Bello and Rich McKay

SURFSIDE, Fla. (Reuters) – Hundreds of fire and rescue workers scoured through tons of rubble on Thursday after a 12-story oceanfront residential building partially collapsed close to Miami, with at least one person dead and 51 still unaccounted for, officials said.

Sally Heyman, a Miami-Dade County Commissioner, said officials have been unable to make contact with 51 people who “supposedly” live in the building, home to a mix of people including families and part-time “snow birds” who spend the winter months in the state of Florida.

Officials said the building, built in 1981, was going through a recertification process requiring repairs and that another building was being newly constructed 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. “The hope is still there, but it’s waning.”

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 a search for survivors.

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.

Emergency responders and officials were still looking for people who might be in the rubble, as well as trying to identify residents not home at 1:30 a.m. (0530 GMT), when an entire side of the building pulled away 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.”

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.

“There were garage underground issues related to that, to make sure that it was done soundly,” Kesl said. “And, to my understanding, there were some cracks from that project – minor cracks – that were just patched up. Nothing, based on my understanding, to the magnitude that would indicate that there was a structural problem that could result in something so catastrophic.”

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 “pancaked” where one floor appears 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.

Eyewitness video obtained by Reuters showed neighbors gathering across the street from the rubble.

“This whole building here, it’s completely gone,” a person can be heard saying.

(Reporting by Marco Bello in Surfside, Florida; Additional reporting by Kanishka Singh and Radhika Anilkumar in Bengaluru, Rich McKay in Atlanta and Peter Szekely in New York; Editing by Jonathan Oatis and Matthew Lewis)