A parking garage under construction in Jacksonville, Fla., partially collapsed Thursday, injuring about two dozen workers, the Associated Press reports.

One person was missing, but police did not know if the worker was trapped or had escaped. Search crews with dogs were looking through the rubble.

The collapse occurred about 6:30 a.m. as construction workers were pouring concrete on the sixth floor of the garage for a condominium complex.

At least 15 people were hospitalized; seven others were treated at the scene, according to WSBTV.com in Atlanta.

"I heard a crack, and then it just crumbled," Rick Caldwell, a construction worker, told The Florida Times-Union.

Mayor John Peyton said at a news conference that the collapse occurred at shift change when workers were arriving and leaving work.

Jacksonville Fire-Rescue officials said the workers injured in the collapse are stable and that none of the known injuries appear to be critical. They are treating the collapse site as a "mass-casualty event."

OSHA is investigating the collapse, according to AP.