Officials in the southeastern U.S. state of Florida said they did not know what caused part of a 12-story beachfront condominium building to collapse early Thursday in a town just north of Miami, killing at least one person and leaving at least 99 others missing.

Miami-Dade Fire Rescue Assistant Chief Raide Jadallah told reporters emergency crews responded to the scene in the town of Surfside about 1:30 a.m. local time. He said the northeast corner of the building, which contained about 55 units, had collapsed. He said rescue crews removed 35 occupants from the intact structure and pulled two others from the collapsed portion of the building.

He said that more than 80 emergency units responded to the scene.

Jadallah said 10 people were treated for injuries at the scene. Surfside Mayor Charles Burkett said two victims were taken to a local hospital where one was pronounced dead. Burkett said he had been told the collapsed section of the building was full of tenants and he expectedthe death toll to rise.

Earlier, Burkett told reporters it was hard to imagine how the apartment complex, built in the 1980s, could collapse, saying, "Buildings don't just fall down."

During a press conference Thursday, President Joe Biden said FEMA was ready to respond and assist in rescue efforts.

"We are ready to move federal resources immediately," he said.

Florida Governor Ron DeSantis visited the site, saying it was "traumatic to see."

"It's a tragic day," DeSantis said at the Thursday afternoon news conference. "We still have hope to be able to identify additional survivors." (VOA, with reports from AP, Reuters and AFP)