The state Department of Environmental Protection is investigating the cause of a thin sheen of oil that appeared this week on the main channel of the Ohio River at Shenango Inc.'s Neville Island coke works and along the five-mile length of the island.

The oily discharge began Wednesday afternoon after a power outage at Shenango from 1:55 p.m. to 3:35 p.m., and was still visible in the river yesterday afternoon.

No fish were reported killed, but Helen Humphreys, a DEP spokeswoman, said the oil discharges were a "violation situation."

"We have to conduct an investigation of what happened and make a determination of the causes and whether the company responded appropriately," Ms. Humphreys said. "But there's no question there's a problem there."

Jim Birsic, Shenango's vice president for health, safety and environmental law, estimated that between 25 and 40 gallons of "wash oil," a light oil used to cool coke oven gas, found its way into sewer lines and then the river from a broken cooling system line.

"When the power came back on, a surge of pressure blew out part of the line in the wash oil cooling system," Mr. Birsic said.