WILLARD, Ohio (AP) _ About 500 people were evacuated when a derailed railroad tank car leaked hydrochloric acid that formed a noxious cloud, but most had returned home Sunday, authorities said.

A tanker containing 20,000 gallons of the acid and two other cars from a CSX freight train derailed in a rail terminal near Willard late Saturday during a routine switching operation, said Ken Schultz of the Ohio Environmental Protection agency.

The spill was contained by 8:15 p.m. by hazardous materials crews who transferred some of the chemical to other cars and spreading limestone, said City Manager Paul Capelle.

Nearly all the evacuees returned home by 9:30 p.m., said Capelle. Ten families who lived downwind of the spill were not allowed to return home, and were being housed at an emergency shelter set up at Willard Junior High School by the Red Cross, said Capelle.

Police Chief David Settig said evacuations of a three-mile area around the site began around 1 a.m. Sunday. He said the acid vapors, smelling like rotten eggs, drifted northward for about 1 1/2 miles from the site of the derailment.

About six people were treated Sunday at Willard Area Hospital for minor eye and throat irritations and headaches resulting from exposure to the fumes, said nursing supervisor Joan Estridge. No one was hospitalized, she said.

The accident's cause was not known, and CSX officials were investigating, said Capelle.

The acid was owned by the DuPont Chemical Co., he said.