WASHINGTON -- The self-described anarchist 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 on Tuesday by the San Francisco Chronicle, set federal authorities scrambling yesterday to tighten security at California airports, which delayed flights and disrupted mail delivery.
But in a piece of mail received yesterday by the New York Times at its New York offices and authenticated by the FBI at its headquarters in Washington late last night, the bomber said the whole thing was a hoax.
"Since the public has a short memory, we decided to play one last prank to remind them who we are," the letter stated. "But, no, we haven't tried to plant a bomb on an airline (recently)."
Nonetheless, the FBI indicated that it would continue to take the threat seriously.
The bomber's boast that the threat to the Los Angeles airport had been merely a "prank" came after a day that saw considerable dislocation at West Coast airports.
Passengers traveling from Los Angeles to Sydney, Australia, were evacuated from a United Airlines jet after crew members found a transistor radio in the rear of the Boeing 747 and suspected it might contain a bomb, Reuters reported.
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."
The bomber in the case -- code named Unabom by the FBI because the initial targets were universities and airlines -- has mailed 16 bombs that have caused 3 deaths and 23 injuries since 1978.
But the threat received on Tuesday broke a pattern: none of the previous bombs were preceded by a warning.
The FBI was "very sure" that the letter to the Chronicle was genuine, said Jim R. Freeman, the special agent in charge of the bureau's San Francisco office and the head of the task force investigating the case.
The Federal Aviation Administration, the Los Angeles police, the FBI and other law-enforcement agents set up operations yesterday at Los Angeles International Airport, which is the world's fifth busiest, handling nearly 1 million passengers a week.
As part of the checks, passengers had to show photo identification and proof that luggage they were carrying was theirs before they could board.
Passengers expressed concern about the bomb threat and minor frustration with the delays, but few canceled travel plans.
"I'm taking my chances," said Allene Golub, 67, as she shuffled through a longer-than-usual security line to check her bag and board a flight for Philadelphia.
In a letter that was part of the packet received by the New York Times yesterday, the writer explained the cryptic reference to a past airliner bomb.
"In one case we attempted unsuccessfully to blow up an airliner," the letter stated. "The idea was to kill a lot of business people who we assumed would constitute the majority of the passengers."
The letter went on: "But of course some of the passengers likely would have been innocent people -- maybe kids, or some working stiff going to see his sick grandmother. We're glad now that that attempt failed."
The explanation was in a passage discussing the morality of past acts. "We don't think it is necessary for us to do any public soul-searching in this letter," it stated. "But we will say that we are not insensitive to the pain caused by our bombings.
"A bomb package that we mailed to computer scientist Patrick Fischer injured his secretary when she opened it. We certainly regret that. And when we were young and comparatively reckless we were much more careless in selecting targets that than we are now."
Yesterday's threat temporarily stopped mail deliveries throughout much of California. All air mail in Northern California was halted, said Horace Hinshaw, a spokesman for the U.S. Postal Service in San Francisco.
Although the authorities resumed air delivery of most letters, 40 additional postal inspectors were assigned to sort through packages, said Dennis Hagberg, a senior postal service inspector in San Francisco. The Postal Service routinely ships mail on commercial and cargo airlines, as do private delivery services.
FBI officials know little for certain about the suspect, although they believe that he lives not far from the state capital, Sacramento.
The little that is known about him is what can be gleaned from his letters to newspapers and from his choice of targets, which in recent years have included scientists, industrialists and lobbyists, and have evinced a fascination with wood and wood products.
The bomber's letters rage against technology and science. After gravely wounding David Gelernter, a well-known professor of computer science, with one of his bombs in 1993, the bomber wrote him a taunting letter, stating: "People with advanced degrees aren't as smart as they think they are. If you'd had any brains you would have realized that there are a lot of people out there who resent bitterly the way techno-nerds like you are changing the world."
In a letter received by the New York Times two months ago, an agenda was set forth for "the destruction of the worldwide industrial system."
Investigators who studied the letter say that while the bomber claims to represent a terrorist group, they believe that his bombs are the work of a loner.