In the waters off San Francisco, the Coast Guard has deployed more than 20 armed and specially trained teams to protect the city's bridges, including the world-famous Golden Gate Bridge.
"We're the first line of defense. We are a Western Wall," said Lt. Sean Kelly of the U.S. Coast Guard. The Sea Marshals' mission is to prevent the hostile takeover of a commercial ship. "It's our job to protect these vessels and the shipping."
The teams of Sea Marshals are the first units of their kind and are particularly crucial now. On Thursday, California Gov. Gray Davis announced that law enforcement officials had "credible evidence" that four California bridges — San Francisco's Golden Gate and Bay bridges, the Vincent Thomas Bridge at the Port of Los Angeles and San Diego's Coronado Bridge — may be the target of an attack.
Recent advances in sea and air technology could make attacks even more difficult to predict or prevent, experts say. "The bridges were built in the '30s. And the vessels between now and when they were built have gotten much larger and much faster," said Coast Guard Cmdr. Craig Brittan.
San Francisco Bay is home to seven bridges, seven ports, a dozen refineries and two international airports.
Recently, the Sea Marshals boarded a Taiwanese freighter, located the captain and secured the bridge area.
"[We] try to prevent somebody to come on the bridge that is not expected to be on the bridge," Brittan said. "Someone they don't know, possibly a stowaway on board, or a crew that was working undercover that got the job specifically to get on board to be a terrorist."
Heightened Alert on West Coast
All along the West Coast, ports are on heightened alert. In Long Beach, that means closer inspection of cargo containers being off-loaded from foreign ships.
Every day, 7,400 containers come into Long Beach. They are screened by a gamma ray machine, which is more sensitive than X-ray. If customs agents spot something suspicious, they break into the container, and unload the cargo for further screening.
Anything coming from the Mideast is getting an even closer inspection than before the Sept. 11 terror attacks on the United States, say officials.
"We are intensifying our inspections on anything going to or coming from those areas," said Jim Furnish, a from U.S. Customs. ABCNEWS' Judy Muller in Los Angeles contributed to this report.