Jacksonville Fire-Rescue spokesman Tom Francis told reporters that workers were pouring concrete on the upper level around 6 a.m. when the structure gave way, WJXT-TV, Jacksonville, reported.

"The good news is that none of the injuries of those people who have been transported ... none of those appear to be traumatic," Francis said. "Most of those are stable, non-critical injuries."

Nine others were treated at the scene for minor injuries.

A WJXT reporter working on an unrelated story across the St. Johns River from the scene heard what he described as sounding like an implosion, then saw a plume of smoke rising from the area.

Rescue workers were trying to clear debris in the search for the unidentified missing worker, officials said.