Agencies Vow to Keep Emergency Measures : Threat: Possibility of hoax will not lessen vigilance, officials say. Scrutiny of passengers, bags, is stepped up.

The thousands of passengers jamming Los Angeles International Airport on Wednesday at the start of the summer holiday crush were confronted with an ominous new worry courtesy of the Unabomber: Is it safe to fly?

That question became a frightening concern after the mysterious bomber threatened to “blow up an airliner out of Los Angeles International Airport.” And it grew even more vexing late Wednesday with a second letter that seemed to indicate a hoax.

Despite the equivocation, federal and local officials said they would not ease the intense security precautions that had been implemented Wednesday. “The FBI is not ready at all to call this thing off,” one city official said.

The bomber’s threat to blow up a plane was called the “most credible” terrorist threat involving Los Angeles International Airport in recent memory and set off a flurry of activity at the world’s fourth-busiest airport. Officials from Mayor Richard Riordan to U.S. Transportation Secretary Federico Pena repeatedly attempted to reassure passengers that every possible security precaution had been put in place to counter that threat.

Authorities required departing passengers to show photo identification and diverted hundreds of thousands of mail parcels to counter the threat of explosives being carried by mail, by baggage or by traveler. Planes that landed even briefly at the airport were cleared of passengers and checked for suspicious packages.

“Everything that can be done is being done to maintain the security at LAX,” Riordan said at an airport news conference. “Every expert who should be on it is on it. We have a great team of experts together working on behalf of the passengers who use LAX.”

Travelers were suddenly faced with security measures that, while extreme for the United States, are standard procedures in many parts of the world that are more familiar with terrorism. Experts said that local arrangements appeared to be appropriate, but they cautioned that technology has not yet presented a fail-safe system for protecting aircraft.

With security still dependent on human scrutiny, the Los Angeles Airport Police Department increased its deployment by about one-third and the Los Angeles Police Department doubled its presence at the airport. The local authorities were joined by a phalanx of other agencies, from the FBI and Secret Service to the Bureau of Alcohol, Tobacco and Firearms, the U.S. Postal Inspection Service and the Secret Service.

Similar but less intense efforts were being made at other California airports, as FAA officials speculated that the threat might extend beyond the target named by the Unabomber in his letter--”an airliner out of Los Angeles International Airport.”

The threat was delivered in a letter to the San Francisco Chronicle. The Unabomber said the bomb would explode within six days, but officials were unsure when that time period began and would not say how long the extra precautions would remain in force. (The letter was postmarked Saturday, but not received until Tuesday.)

Given the mysterious bomber’s previous methods, which have killed three people and injured 23 over 17 years, officials focused much of their attention on diverting potentially dangerous mail.

Authorities decided that no parcels weighing more than 11 ounces would be permitted on flights out of any California airport. An estimated 200,000 packages move through Los Angeles International Airport every day. Postal officials in Los Angeles were considering whether to deliver the packages by truck or divert them to a special screening center.

The center was being hastily assembled at a hangar at the south end of the airport with about a dozen X-ray machines to be staffed by dozens of technicians from various law enforcement agencies. “It is going to be a very painstaking process,” said one airport official.

The Unabomber’s unusual behavior--making a highly public threat--and conflicting statements made his next step impossible to anticipate. As a result, precautions extended to the expected 150,000 pieces of luggage that were scheduled to be shipped out Wednesday on about 1,000 flights.

Airline and security officials launched a massive “passenger-bag matching system,” familiar to many international fliers, but a relative novelty domestically.

The system is designed to assure that all bags placed in the holds of planes are matched to a passenger on board--under the theory that most terrorists are not bent on taking a suicide flight with their bomb.

Passengers arriving at the airport Wednesday had to present photo identification when they checked in, in an effort to assure that those named on tickets and baggage tags were the same people standing at the counter. As flights prepared to leave, passenger lists were scrutinized for “missing” passengers. If those passengers did not take their seats, their flights would be held and their bags removed, airport and airlines officials said. No such special holds occurred Wednesday.

Tim Neale, a spokesman for the Air Transport Assn., said the bag matching system “is a very successful security measure internationally.”

At the international airport in Frankfurt, Germany, officials go so far as to require passengers to descend to the Tarmac and personally point out their bags before they are placed on board.

That is typical of the stringent measures found overseas, said John Horn, a senior managing director with Kroll Associates, the world’s largest security management and investigations company.

At airports in London or Tel Aviv, for instance, passengers are confronted on a daily basis with precautions at every step of the registration and boarding process that are rarely seen here. Passengers are required to carry a photo ID that is checked against their luggage. They are then asked a series of questions such as “When did you pack your bag? Did anyone help? Has it been out of your sight since you packed it?”

During the process, passengers are also compared to computer profiles of terrorists, Horn said, and even after boarding, flight attendants are trained to closely scrutinize passengers who might match terrorist profiles.

Baggage on international flights originating in this country and overseas are routinely subjected to X-rays. Bags on domestic flights in the United States are not usually X-rayed, but starting Wednesday those fitting a “profile” linked to the Unabomber were scanned.

“We don’t have the capability of screening [all] domestic bags by X-ray,” said one airline executive who asked not to be identified. “We just don’t have the equipment to do it domestically although we do it internationally. It would grind the system to a halt here.”

He said the bag matching system at the airport is the best alternative short of examining each bag.

Because of the extra time for the identification checks and other precautions, passengers were asked to arrive at the airport two hours before their flights. In addition, the FAA had alerted airlines about a “profile” of bags to handle with suspicion, and those will be X-rayed or subjected to some other form of inspection.

Even those precautions could be breached, though, if passengers are not careful to protect their baggage. To make the system more foolproof, airline officials urged passengers not to carry items onto planes for strangers.

“Passengers shouldn’t take something from someone who is just standing out on the ramp and says, ‘Can you take this to my grandmother for me,’ ” Neale said. “They absolutely shouldn’t deal with someone like that, no matter how persuasive they are.”

Despite all the extra activity, airline officials said the regular traffic of about 2,000 flights a day should continue. No substantial delays had been reported by late Wednesday.