At least 102 people have now been accounted for, but it is uncertain how many were inside when it came down.
Champlain Towers South was completed in 1981. When it collapsed it was going through a recertification process for 40-year-old buildings, in line with city safety regulations.
As the building had stood since 1980, it was due to its standard 40-year review. The building was undergoing its “recertification” process and required repairs, officials said.
President Joe Biden has approved an emergency declaration for Florida, meaning the Federal Emergency Management Agency (FEMA) will help state agencies with the relief effort.
Eyewitnesses described hearing what sounded like thunder before seeing a huge cloud of dust in the aftermath of the collapse. One compared the scene to the 11 September 2001 attacks on the Twin Towers in New York.
A study from researchers at Florida International University published last year found that the building had been sinking at a rate of two millimeters per year in the 1990s, which may have affected the building structurally.
A full investigation will begin after the rescue mission.