MIAMI, Florida — A massive search and rescue effort is underway after part of a 12-story residential building collapsed early Thursday in the South Florida town of Surfside, leaving dozens unaccounted for.

One person is dead, Surfside Mayor Charles W. Burkett said. Two people have been pulled from the rubble at Champlain Towers South since the collapse happened around 1:30 a.m., Miami-Dade Fire Rescue Assistant Chief Ray Jadallah said, without addressing their conditions.

About 55 of the building's 136 units collapsed, Jadallah said, leaving huge piles of rubble and materials dangling from what remained of the structure in the beachfront community a few miles north of Miami Beach.

Rescuers helped a boy from the debris alive, a witness said, and video showed responders helping others leave the standing portions of the building, sometimes using a bucket atop a fire truck's ladder.

"We are going to do everything we can possibly to identify and rescue those who have been trapped in the rubble," Miami-Dade County Mayor Daniella Levine Cava said Thursday morning.

Besides the two pulled from rubble, 35 others had been helped from standing portions of the building, Jadallah said.

At least 51 people were unaccounted for as of around 10 a.m., having been reported missing to the county's emergency reunification hotline, Miami Dade County Commissioner Sally Heyman said.

Surveillance video obtained by local Fox Sports radio anchor Andy Slater appears to show the collapse: A huge section of the building fell first, followed by another portion about nine seconds later.

A resident on the third floor, Barry Cohen, heard what he thought sounded like an explosion. His apartment was intact, but when he opened his door and tried to leave, he "looked down the hallway ... and there was nothing there," he said.

"It was just a pile of dust and rubble," and the building shook as he awaited rescue, Cohen said.

After about 20 minutes, a rescue crew used a cherry picker to help him, his wife and another resident from a balcony, he said.

Florida Gov. Ron DeSantis said officials are hoping for additional rescues, but "we are bracing for some bad news, just given the destruction we're seeing."

Besides two people who were taken to hospitals, 10 were assessed and treated at the scene, Jadallah said.

The cause of the collapse wasn't immediately known. The building was undergoing roof work, but it's unknown whether this was a factor in the collapse, Burkett said. — CNN