CINCINNATI — Construction employees were back at work Wednesday on most of the $400 million Cincinnati casino project for the first time since a floor collapse injured 13 workers last week and prompted a shutdown of the construction.

The federal Occupational Safety and Health Administration is leading the investigation of what caused the collapse, which occurred on Friday morning as workers were pouring concrete in a 60-foot by 60-foot, second-floor area referred to as a bay.

That bay remains shut down for the investigation. By law, OSHA has up to six months to issue a report of its findings, said Scott Allen, spokesman for the U.S. Department of Labor, of which OSHA is a part.

Investigators are focusing on what caused a steel beam to fall, dropping a corrugated steel floor and the employees who had been working there.

Messer Construction Co., the general contractor, isn’t commenting on what may have caused the accident, deferring to OSHA officials.

Project officials said they still hope to have what will be the Horseshoe Casino open in spring 2013.

One of the 13 injured workers remains hospitalized and was listed in serious but stable condition in Bethesda Hospital North, near Cincinnati. Authorities haven’t released his name.