California officials declared a super-heightened state of alert this week after receiving a threat that someone planned to commandeer an airplane and fly it into the Golden Gate Bridge. Traffic flowed as usual over the span, but the California Highway Patrol, the U.S. Coast Guard, and other agencies were on patrol, looking for signs of trouble. A foreign government warned the U.S. that the towering suspension bridge and other landmarks, including the Brooklyn Bridge, could be targeted. There have been more than a half dozen threats against the Golden Gate since Sept. 11. For us, theyre all serious, said Mary Currie, a spokeswoman for the Golden Gate Bridge Highway and Transportation District.