Federal Occupational Safety & Health Administration investigators are looking into the collapse of a Jacksonville, Fla., parking garage under construction while rescue workers and search dogs swarm the site hoping to locate a missing worker. OSHA officials were unaware of a local press report on a previous fall-related injury at the site, and a contractor spokesman declined to either confirm or deny it.

Multimedia: Video: Jacksonville Parking garage Collapse

The structure collapsed four hours into a concrete pour on the sixth and top level of a downtown parking garage early Dec. 5, sending 23 workers to area hospitals.

The partial, pancake-style collapse occurred shortly after 6 a.m. on the Berkman Plaza 2 parking garage, being built by Choate Construction Co., Atlanta, for Harbor Companies, also of Atlanta.

Jacksonville Fire and Rescue reports transporting 14 people to area hospitals in rescue vehicles and nine by bus. Evin Willman, spokesperson for developer Harbor Companies, says 13 people were taken to hospitals by ambulance, with two in serious condition, two in good condition, and the rest suffering minor injuries. One hundred firefighters, with 25 pieces of equipment responded.

"This is probably one of the largest responses Jacksonville Fire and Rescue has engaged in in the last five or six years," says John Bracey, public information officer for Jacksonville Fire and Rescue.

Concrete subcontractor A.A. Pittman & Sons Concrete of Jacksonville began the pour at 2 a.m. Irene Pittman, who answered the phone at the firm, declined to comment, except to ask people to pray for the family of the missing man.

Jacksonville Fire and Rescue Operations Division Chief Brady Rigdon said during a press conference that the department will continue the slow and methodical process of searching for the missing man until they are sure no one is in the building.

One fire department canine search and rescue (SAR) team made four trips to the debris pile on Thursday. Additional SAR dog teams from Orlando and Tampa also are searching the site. Crews began removing concrete at about 1 p.m. Willman describes it as a slow process, which will involve thousands of pounds of concrete. Bracey estimates it will take several days to complete the operation.

In a written press statement, William Millard Choate, president of Choate Construction, said, "Upon completion of rescue operations, there are teams of professionals from all over the country assembled to begin this investigation."

James D. Borders, Occupational Safety & Health Administration's Jacksonville area director, confirmed the agency was investigating the cause and whether there were any violations of OSHA regulations.

"We have no idea what happened," Borders says.

Despite a local news report of a prior fall-related injury at this job site, Borders says he is not aware of a previous incident and if a problem occurred, OSHA did not investigate it. David Page, spokesman for Choate, would not confirm or deny that incident happened.