U.S. prosecutors have opened a criminal investigation into a cargo ship crash that caused a massive oil spill in San Francisco last week.

The head of the U.S. Coast Guard, Admiral Thad Allen, told reporters in San Francisco, California, Sunday his agency is working with the U.S. Attorney's office to determine the cause of the crash.

A preliminary Coast Guard investigation found that human error, not mechanical failure, caused the ship to hit a bridge last Wednesday.

The accident did not damage the bridge, but the impact opened a 30-meter gash in the vessel's hull, puncturing two fuel tanks and spilling more than 200 thousand liters of thick bunker oil into San Francisco Bay.

U.S. Senator Dianne Feinstein of California surveyed the damaged areas Sunday. She said the system for responding to spills needs to be improved.

Admiral Allen defended the Coast Guard's response to the incident, citing poor visibility due to fog at the time of the crash. He said preliminary information suggests it took time to figure out the extent of the spill because equipment used to measure fuel was damaged in the crash.

The National Transportation Safety Board, an independent federal agency, arrived in San Francisco Sunday to launch its own investigation.

California has ordered an emergency cleanup of the bay and its oil-coated shoreline, and nearly 800 people are working to rescue birds, fish and other wildlife from potentially deadly pollution.

Coast Guard officials are checking communications between the ship's Chinese crew, the American harbor pilot who was on board the Cosco Busan, and the local Coast Guard station monitoring ship traffic.

By late Sunday, cleanup crews had recovered about one-third of the spilled oil, but the effort is expected to continue for weeks or months.

Some information for this report was provided by AP.