填空题

When in August last year Britain’s security services uncovered an alleged plot to smuggle the liquid components of a bomb onto an aircraft at Heathrow, the world’s busiest international airport almost ground to a halt as additional passenger checks were ordered. It was not just flights starting from Heathrow that were delayed or cancelled, but many incoming services too. In the following days British Airways alone cut more than I 200 flights.
Intelligence to prevent attacks is part of what experts call a "layered" security approach. Other layers include checking identities, scanning people and their luggage and searching them at random. Another and increasingly important one is to see how people behave. "There are identifiers of people who have hostile intent that you can pick up," says Kip Hawley, the head of America’s Transportation Security Administration. "Our testing indicates an extraordinary high degree of success."
Security officials with the Israeli airline El A1 already spend a long time questioning passengers in order to identify behavioral traits that mark them out as dangerous. But El A1 is small and America’s airports would grind to a halt if such time-consuming procedures were adopted everywhere. So the system that will emerge in America is likely to involve pre-screening of passengers right from the time they make their reservation. Then, as they go through the different layers of security at the airport, some passengers will be asked questions by specially trained staff who are looking out for things like involuntary facial-muscle movements. A twitch (颤搐) is impossible to hide. "The more you try to avoid it, the more pronounced it becomes," says Mr. Hawley.
Machines used to scan luggage at airportsare getting more powerful too. Instead of just alerting staff to suspicious images, newer scanners will be able to work out what they show. New biometric passports, which contain details such as fingerprints and iris(虹膜) scans, will also improve identification.
But Mr. Hawley insists that security measures should not rely solely on machines however they are sophisticated, as threats change. It will take a combination of science and technology to reduce the hassle(麻烦) involved in passing through airports. El Al’s security procedures can’t be widely applied in America because they are ____.