Federal officials tightened security at California airports today after a threat, purportedly bythe elusive Unabomber,to blow up an airliner at Los Angeles International Airport sometime in the next six days. The threat appeared in a short letter received Tuesday by the San Francisco Chronicle. "WARNING," it reads. "The terrorist group FC, called unabomber by the FBI, is planning to blow up an airliner out of Los Angeles International Airport some time during the next six days. To prove that the writer of this letter knows something about FC, the first two digits of their identifying number are 55." The FBI saysthe letter resembles others from the mysterious serial bomber,who has been blamed for 16 mail bombings since 1978 that killed three people and injured 23. In an unusual -- perhaps unprecedented -- warning, the Federal Aviation Administration warned California airline passengers to be alert to suspicious bags, parcels and other items.