Federal authorities are keeping security tight at California airports despite a new letter from the Unabomber that says his threat to blow up a plane out of Los Angeles was just 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)." The serial mail bomber,who since 1978 has mailed or planted 16 package bombs that killed three people and injured 22, also offered an apology of sorts for the past:"We don't think it is necessary for us to do any public soul-searching in this letter. But we will say that we are not insensitive to the pain caused by our bombings." The joke was lost on the state of California, where thousands of postal workers combed through nearly every parcel and wary airline passengers -- if they ventured out at all -- were forced to stand in long lines and present identification. Officials, though relieved by the letter, were still posting new officers at Los Angeles International Airport, which handles nearly 1 million passengers a week.