A Deceptive Killer

Few things so deadly have ever looked so innocent. They have the appearance and consistency of soft taffy and can be molded, stretched or cut into any shape. They burn so safely that American G.I.s in Viet Nam used them as emergency cooking fuel. Yet plastic explosives pack roughly twice the force of an equivalent amount of dynamite. Many nations, including the U.S., produce them for military purposes. But large amounts have made their way into the hands of terrorist groups around the world, posing a fiendishly difficult problem for airline security. Because the explosives can be so easily formed into innocuous shapes, they can pass undetected through security checks. The deadly plastic is also odorless and cannot be sniffed out by trained dogs.

The Federal Aviation Administration has been working with two U.S. companies to solve the problem. Starting next summer, the FAA will receive five new devices developed by San Diego-based Science Applications International Corp. for screening checked luggage. The machine bombards luggage with neutrons that interact with the nitrogen in explosives, touching off a characteristic pattern of gamma rays. In tests conducted last summer at Los Angeles and San Francisco airports, the devices spotted more than 95% of suitcases containing test samples of explosive materials. But because they employ dangerous radiation, the machines, which cost as much as $1 million each, cannot be used on passengers.

That is not a problem with the system designed by Thermedics in Woburn, Mass. It uses jets of warm air to collect vapors given off by either luggage or the clothing of passengers, who would be required to step into a three- sided booth. The vapors are then subjected to six different computerized chemical tests that together take about 25 seconds. In a five-day trial run at Boston's Logan Airport last October, the system, which would cost roughly $250,000, nabbed 50 out of 50 test samples sent through.

Another technological approach would not prevent bombings, but it could help identify those who commit them. Explosives can be chemically "tagged" so that telltale traces can be used to determine their origin after a blast. If producer nations could agree on a tagging system for military explosives, it would increase the chance that future terrorists might be tracked down and brought to justice.