Federal officials tightened security at California airports today afterthe elusive Unabomber threatened to blow upan airliner at Los Angeles International Airport sometime in the next six days. "We are taking it very seriously," Federal Aviation Administration official Ed Jones told a hastily-assembled news conference at the airport. Seeking to calm thousands of nervous passengers, Los Angeles Mayor Richard Riordan added: "My judgment is, I would fly. But it's a credible threat." FBI officials said they were "100 percent sure" that a short letter received Tuesday by the San Francisco Chronicle came fromthe mysterious serial bomber, who has been blamed for 16 mail bombings since 1978, the last in April. The Federal Aviation Administration warned California airline passengers to be alert to suspicious bags, parcels and other items.