The Unabomber, whose mail bombs have terrorized executives and researchers for 17 years, has threatened to blow up an airliner flying out of Los Angeles International Airport within six days.
The threat, made in a letter received by the San Francisco Chronicle, set federal authorities rushing to tighten security at California airports Wednesday, which delayed flights and disrupted mail delivery.
However, late Wednesday the New York Times said it also had received a letter in which the Unabomber took responsibility for the bomb threat and said the whole thing was a ruse.
"Since the public has a short memory, we decided to play one last prank to remind them who we are," the Times' letter said. "But, no, we haven't tried to plant a bomb on an airline."
The Times said the FBI authenticated the mail late Wednesday.
After receiving the threat, however, airlines were taking extra precautions. Passengers were evacuated Wednesday afternoon from a United Airlines flight that landed in Sydney, Australia, after crew members found a transistor radio in the rear of the 747 and suspected it might contain a bomb. The flight originated in Los Angeles.
At Tampa International Airport, which has an average of five flights daily to or from Los Angeles, officials would say little about security measures.
"We're maintaining our usual security profile," said Brian Rumble, assistant director of operations.
In his letter to the Chronicle, the person claiming to be the bomber, wrote: "WARNING. 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."
In a letter mailed in April to the New York Times, a person claiming to be the bomber asserted that FC was an anarchist group conducting terrorist actions "to promote social instability in industrial society."
The writer said that he wanted to tell his story and was working on an article of between 29,000 and 37,000 words "that we want to have published." The newspaper said it would determine what action to take when and if such a manuscript was made available to the New York Times.
The Unabomber _ code-named by the FBI because the initial targets were universities and airlines _ has mailed 16 bombs that have caused three deaths and 22 injuries since 1978.
The FBI is certain that the threat in the letter to the Chronicle was genuine, said Jim Freeman, the special agent in charge of the bureau's San Francisco office and the head of the task force investigating the Unabomb case.
When asked how sure he was that the letter was from the Unabomber, Freeman said: "Very sure. A hundred percent."
The FAA, the Los Angeles police, the FBI and other law-enforcement agents set up operations Wednesday at Los Angeles International, which handles nearly 1-million passengers a week.
Many flights were delayed for up to 30 minutes or more throughout California as authorities established checkpoints to verify the identities of all passengers. They also X-rayed all luggage.
The threat temporarily stopped mail deliveries throughout much of California. All air mail in Northern California was halted.
_ Information from Times correspondent Mike Mahan, the San Francisco Chronicle and Reuters was used in this report.