Updated

A preliminary investigation found human error caused a cargo ship to crash into the Bay Bridge, leading to San Francisco Bay's worst oil spill in nearly two decades, the U.S. Coast Guard said Saturday as rescue teams raced to save hundreds of seabirds.

Coast Guard officials said their early probe was focused on communication problems between the ship's crew and the pilot guiding the vessel, as well as communications between the pilot and the Vessel Traffic Service, the Coast Guard station that monitors the bay's shipping traffic.

Officials would not detail what evidence was uncovered. But Coast Guard Cmdr. Andrew Wood said "the mere fact that they collided with a fixed object" offered clear evidence that a communication problem had occurred.

The Cosco Busan was headed out of the bay when it sideswiped a support on the San Francisco-Oakland Bay Bridge Wednesday morning, leaving a gash nearly 100 feet long on the side of the 926-foot vessel.

The crash ruptured two fuel tanks, which leaked about 58,000 gallons of heavy bunker fuel into the bay — the bay's worst oil spill in nearly two decades.

On Saturday, the Coast Guard increased the number of ships to 20 from 11 the previous day to work on skimming the oil from the bay. Nearly 20,000 gallons of oily liquid had been sucked up by Saturday morning, said Petty Ofc. Sherri Eng.

About 500 workers joined shoreline cleanup crews to mop up the damage — a job that is expected to last weeks or possibly months.

Coast Guard officials said communication between the vessel's pilot, Capt. John Cota, and the ship's all-Chinese crew was likely a factor in the crash, but that the language barrier was not an issue since the ship's captain and officers are required to speak English.

Cota, who is American, is among a group of specially trained pilots who are not members of a ship's crew but typically come on board to maneuver large cargo vessels in San Francisco Bay.

Rear Adm. Craig Bone, the Coast Guard's top official in California, declined to comment on a report that the Coast Guard had warned the pilot about the ship's course shortly before the crash.

John Meadows, the lawyer for the ship's pilot, told The San Francisco Chronicle that the nearby Coast Guard facility radioed Cota and questioned his bearings, but that the pilot immediately responded saying the ship's instruments showed he was on the correct heading.

Bone acknowledged that there were communications between the ship and the Coast Guard's traffic facility before the collision. He said the communications involved the ship's course and speed but declined to comment further on the nature of the communications.

Authorities said most of the spilled oil will never be captured and eventually will dissolve into the water. Concentrated globules could remain for months and cause problems for seabirds.

At least 60 birds were found dead while 200 live birds were recovered and sent to a rehabilitation center in Solano County.

State wildlife officials said they have received hundreds of reports of oiled birds found on Bay area beaches, two dozen of which were closed after tides carried the oil under the Golden Gate Bridge and into the Pacific Ocean.

Most of the injured birds are surf scoters, a duck-like species that lives on the water's surface and dives for fish. When oil gets on their feathers, it disrupts their waterproofing system and ability to stay warm, forcing them on shore where they are at risk of starvation.

"Oil and feathers don't mix," said Yvonne Addassi, a wildlife biologist with the state Department of Fish and Game. "We're in a time crunch. The birds can only stayed oiled for so long before they can no longer be rehabilitated."

Fish and Game officials said the fuel generally stays on the water's surface, and they had not seen any evidence that fish have been harmed — though they are concerned that the spill could affect bay herring that spawn at this time of year.

Gov. Arnold Schwarzenegger declared a state of emergency Friday after meeting with state, federal and local officials overseeing the cleanup. The proclamation makes additional state personnel, funding and equipment available.