Federal authorities are keeping security tight at California airports despite a new letter from the Unabomber that sayshis threat to blow up a plane leaving Los Angeleswas merely a hoax. "Since the public has a short memory, we decided to play one last prank to remind them who we are," reads a letter sent to The New York Times and authenticated by the FBI last night. "But, no, we haven't tried to plant a bomb on an airline (recently)." His joke was lost on the state of California, where thousands of postal workers continued to comb through parcels, diverting most airmail packages to ground delivery, while wary airline passengers were forced to stand in long lines and present identification. Los Angeles International Airport handles nearly 1 million passengers a week. Police experts believethat the Unabomber envies the publicity surroundingthe Oklahoma City bombing. The letter in which he threatened to blow up a plane was mailed on Monday, the day Timothy McVeigh appeared on the cover of Newsweek.