A Greek tanker that spilled up to half a million gallons of crude oil into the Delaware River last month apparently gashed its hull on a discarded pipe protruding from the river bottom, the Coast Guard (search) said Tuesday.

Sonar tests on Saturday located the 15-foot, U-shaped pipe. Investigators found gouges on it and traces of paint that matched the ship, officials said.

The rusty pipe, made of cast iron, stuck out about three feet from the river bottom and was about 700 feet from the water's edge, roughly the same distance from land where the Athos I (search) began gushing oil after an uneventful trip from Venezuela.

"We don't know what the origin of the pipe is," Coast Guard Capt. John Sarubbi said. "We need to get the pipe up and do some further examination."

The pipe had a diameter of about 3 feet and had two broken brackets on it, suggesting it had been torn from its original location, he said.

The Nov. 26 spill killed wildlife and fouled 70 miles of the river. The cleanup is expected to cost millions.

An undetermined amount of oil was released. Some 473,000 gallons of crude are unaccounted for, although investigators believe much of it may still be in compartments inside the ship.

Other vessels had passed over the same site without incident, and authorities said they did not know why the Athos I hit the pipe. But they said the crew was handling the ship properly at the time.

The pipe was found in a federally controlled area that is supposed to accommodate ships with drafts of up to 40 feet. The Athos I had a draft of 361/2 feet.

"We always believed that our crew acted appropriately, and hadn't made any errors," said Michael Hanson, a spokesman for the company that operated the ship, Tsakos Shipping & Trading.

Regardless of the cause of the spill, the company is legally obligated to pay initial costs of the cleanup.

The Army Corps of Engineers (search) is responsible for keeping the area free of silt and dangerous objects. The last survey, concluded in June, found no sign of the pipe, corps spokesman Merv Brokke said.

Meanwhile, the U.S. attorney in Newark, N.J., has opened an investigation into whether someone could be held criminally responsible for the spill, Sarubbi said.

The tanker was being maneuvered by tugboats as it tried to dock at the Citgo Petroleum Corp. (search) refinery in West Deptford Township, N.J., when the pipe tore two gashes in its hull, one of which was six feet long.