SAN FRANCISCO -- A 'single source' in Europe has told U.S. officials that there may be a terrorist attack this weekend at a waterfront oil facility in either San Francisco, New York City or Los Angeles.

While the tip has not been corroborated by another source, the FBI is taking the threat seriously and has issued a terror alert to Bay Area authorities. An additional target, the source told federal officials, may be the Boulder Dam.

The California Highway Patrol told KTVU that it had not added any additional officers to its patrols, but was keeping a close eye on the local bridges, chemical plants and oil refineries.

The Richmond Fire Department did confirm that it was stepping up its patrols of the Chevron Refinery, but a spokeswoman at the Contra Costa Fire Department -- which has the Tosco Refinery complex within its jurisdiction -- said she had not been told about the alert.