What if the government decides that a traveler has not been misidentified and belongs on its watch list? According to the agency, there is no formal appeals process. Travelers can either accept the decision or, as a last resort, sue the government.
That is exactly what several travelers did in April, when they filed a class-action challenge to the watch list with the assistance of the American Civil Liberties Union. The A.C.L.U.'s lead lawyer in the case, Reginald Shuford, said airline passengers were being subjected to repeated delays or detentions and had ''no way to clear their names.''
Mr. Shuford said the watch list violated passengers' constitutional rights. ''There's no mechanism in place that would allow travelers to avoid the repeated humiliation of being singled out as a suspect when they try to fly,'' he said. ''The watch list is a complete mess. Its purpose may be laudable, but the way it's currently administered and maintained does not achieve its purpose.''
Critics of the current screening system agree that the watch list is defective and that frequent travelers are suffering because of it. ''The list is obsolete,'' said Richard Eastman, a technology consultant in Newport Beach, Calif. ''There are more efficient ways of finding a threat.''
He suggested that a smarter way to catch suspicious passengers would be to identify them before check-in, which was the idea behind Capps II, a proposal to prescreen airline passengers that has been abandoned, and its successor, a modified concept called Secure Flight. Under the new program, the responsibility for checking airline passengers' names against the watch lists will shift from the airlines to the security agency, which the agency says will eliminate most of the false alerts caused by what it calls the ''current outdated system.''
Detractors also maintain that the secrecy shrouding the watch list is pointless. ''How do you get on the list?'' asked Edward Hasbrouck, a privacy advocate in San Francisco. ''Nobody knows. How do you get off the list? Nobody knows.''
Ms. Stover of the security administration said passengers were added to the secondary list if there was ''a pattern in something they have done in the past that merits future scrutiny,'' and were put on the no-fly list only if they were wanted ''for activities that may be terrorist-related or pose a threat to national security.'' She declined to elaborate.
But Mr. Hasbrouck said even if the government could successfully argue for keeping its reasons secret -- and he doubted it could -- there were still no legal safeguards to prevent innocent passengers from being erroneously added to the watch list. ''The model is flawed,'' he said. ''People should only be placed on the list based on an order from a court of competent jurisdiction following an adversarial evidentiary hearing. The burden of proof should be on the government to show that someone is dangerous, not the other way around.''
He is hardly the only one frustrated by the lists. Frequent travelers like Heather Marie Owens, a corporate travel agent in Norfolk, Va., are so fatigued by their watch-list problems that they have resorted to extraordinary measures.
''On every recent flight I have flown on since November 2001, they tell me I'm on a watch list,'' she said. ''When I ask why, they say it's random. But I don't believe them anymore.''
Ms. Owens said she had endured increasingly humiliating searches by security screeners and had experienced a range of emotions from bewilderment and helplessness to, finally, resignation. On her next trip to Miami, she is bypassing the security agency altogether.
''This time,'' she said, ''I'm driving.''