CINCINNATI -- A floor collapsed into a V shape Friday at the construction site of a new Ohio casino, sending workers sliding to the ground, leaving one worker with serious injuries and hurting at least a dozen others.

The collapse occurred shortly before 8 a.m. as a crew was pouring a section of concrete floor at the Horseshoe Casino Cincinnati site, Steve Rosenthal, of co-developer Rock Gaming LLC, said Friday. It came just weeks after a similar accident at a Cleveland casino with the same developers.

The worker with serious injuries was downgraded from fair condition at a Cincinnati hospital Friday night.

Authorities had said earlier that there were no life-threatening injuries.

Shouts could be heard on a 911 call right after the collapse as the caller told the dispatcher, "We've got one guy that's in desperate position."

"There's about 20 guys just fell through the floor pour. You've got to get down here as quick as possible. ... They fell about 15, 20 foot. Hurry," he said.

"We've got men under a beam as well. We got to get this beam off of them."

Rosenthal said at a news conference that it was too soon to determine what caused the collapse.

Fire Chief Richard Braun, who was one of the first on the scene after the collapse, said a beam supporting the floor "sheared away" and the floor came down while the workers were on top of it.

"They basically rode the V down," Braun said. No one was underneath the 60-foot-by-60-foot section of floor.

The injured were sent to hospitals with what appeared to be mostly bruises and bumps, and possibly some broken bones, the fire chief said. All workers were accounted for, Rosenthal said.

The man in serious condition is at Bethesda North, and is the only worker from the collapse at the hospital, said Joe Kelley of TriHealth System. The system's Good Samaritan Hospital treated and released two workers.