David Swanson, The Philadelphia Inquirer via AP Clean-up workers and environmental experts pass through oil swirling on the surface of the Delaware River across from Philadelphia International Airport. The oil slick now stretches 40 miles. Click photo for larger image.

PAULSBORO, N.J. -- The Coast Guard says investigators believe much more oil spilled from a ruptured tanker in the Delaware River near Philadelphia last week than the 30,000 gallons initially reported.

Coast Guard Lt. Buddy Dye said 473,500 gallons from the tanker Athos I are unaccounted for, but called a leak of that size a "worst-case scenario."

The worst oil spill in the river's history occurred in 1989, when a tanker ran aground near Claymont, Del., spilling 300,000 gallons.

Dye said it could take four or five days for investigators to probe the vessel to determine how much oil actually escaped through a gash in the hull. The Coast Guard said the cause still was unclear.

The initial estimate of 30,000 gallons might have been inaccurate because it was based on a reading taken while the ship was listing 4 or 5 degrees to the side, Coast Guard Chief Petty Officer Patrick Montgomery said.

"This is a complete mystery to us," he said. "We have people scratching our heads."

New Jersey Environmental Protection Commissioner Bradley M. Campbell said that while the oily muck was concentrated within a few miles of Paulsboro, across the river from Philadelphia International Airport, an oily sheen could be seen along about 40 miles of the river.

New Jersey's two senators said they would push for a law compelling owners of similar tankers to switch to more secure vessels.

After flying over and then boating through parts of the Delaware affected by the spill, U.S. Sens. Jon Corzine and Frank Lautenberg said they would introduce a bill to increase the liability for ship owners in case of a leak.

After the 1989 Exxon Valdez disaster in Alaska, federal lawmakers agreed to ban from the United States by 2010 single-hulled oil tankers such as the one that was torn open on the Delaware. But Corzine and Lautenberg, both Democrats, said the oil shipping companies are moving slowly to comply.

"We're not going to let them wait until the last minute and hope for a recasting of the date," Lautenberg said.

The legislation the two said they would introduce would double the amount ship owners are required to pay to clean up a disaster. The senators said the measure should speed compliance by shipping companies.

According to the current formula, Tsakos Group, the Greek company that manages the Athos I, could be made to pay up to about $45.5 million toward cleaning up the spill and mitigating the environmental and economic problems it may cause.

Corzine said it is not clear whether that amount may be enough in this case; officials say it will take more investigation to determine the cost of fixing the problems created when the Athos I leaked.

A federal oil pollution fund is available to pay costs beyond the current liability cap.

"It's all going to be cleaned, every twig, every grain of sand," said Paulsboro Mayor John Burzichelli, a Democrat who also serves on the state General Assembly.

The Army Corps of Engineers completed a sonar survey of the river without finding anything in the water that would strike the vessel, Montgomery said Tuesday. He also said there was no evidence of a navigation error that might have led the Athos into shallow waters or caused it to bump up against something on the shore or in the water.

In some spots, the odor of the oil was powerful; elsewhere, the river smelled like the lemon-scented detergent that some 600 workers are using for the clean up.

Campbell said the oil was within three miles of a drinking-water intake in Delran and about a dozen miles from the nuclear power facility in Salem _ close enough that officials have asked the companies that run those utilities to take extra precautions.

"A tablespoon of oil can create up to a football field of sheen," Campbell said.

Birds that come into contact with even the smallest amount of oil can lose their ability to float and to regulate their body temperatures. Birds that are covered with oil typically sink after they perish.

Campbell said two nesting pairs of bald eagles have been observed to have been splattered with oil. He said the eagles are not incapacitated and that wildlife managers are considering trying to capture and clean them.

Two of the eagles nest on an island in the Delaware and two nest along the Mantua Creek, one of several tributaries affected by the spill.

Maya K. van Rossum, riverkeeper for the Delaware Riverkeeper Network, said the problems caused by oil in streams can be even more severe than on the river because a little oil in a small body of water can mean a relatively high concentration.

The effects of the spill are not just environmental, but also economic. The Philadelphia region is among the nation's largest port areas.

Yesterday, after days of being halted, some cargo ships were being allowed to traverse the oil-laden areas.