The barn roof had already collapsed by the time firefighters arrived at the Warren County Fairgrounds, about 25 miles northeast of Cincinnati, according to Capt. Krista Wyatt of the Lebanon Fire Department. The fire was reported about 5 a.m.
The bodies of two men were pulled from the barn and were badly burned, said Shane Cartmill, a spokesman for the Ohio State Fire Marshal. Officials were trying to identify the men.
Lamar Moody, lead trainer at Lebanon Raceway, a harness racing track based at the fairgrounds, said two of his employees were not accounted for.
No one was supposed to be in the stables overnight and it was not clear if the two men killed were working early or sleeping in the barn.
"There was a sign on the building that said people we not supposed to sleep in there," said Victor Gray, a Lebanon resident who lost three horses in the fire.
The cause of the blaze was under investigation.
Horse owners stood stunned as they watched heavy machinery peal back the metal roof of the collapsed structure so firefighters could sift through the debris and extinguish what remained of the blaze. The barn was about the length of a football field.
In 1988, a fire in a similar part of the county-owned racetrack and fairgrounds killed 35 horses. No one was killed in that fire.