WASHINGTON, April 22 — Civil rights advocates demanded today that the federal government explain how hundreds of people — some of them vocal critics of the Bush administration — have ended up on a list used to stop people suspected of having terrorist links from boarding commercial air flights.
In a lawsuit filed in San Francisco, the American Civil Liberties Union said government officials had improperly withheld information about how people wind up on the "no fly" list, what steps are taken to ensure its accuracy and how people who are erroneously detained at airports can get their names off the list.
"Without even basic information about the no-fly list or other watch lists," the lawsuit said, "the public cannot evaluate the government's decision to use such lists."
Since the attacks on Sept. 11, 2001, the F.B.I. and federal transportation officials have generated secret lists of people suspected of having terrorist ties who should be stopped and questioned if they try to board an airplane.
Law enforcement officials say the policy is a necessary safeguard to prevent the type of security lapses that allowed two of the Sept. 11 hijackers to board a plane even though intelligence officials had reason to suspect they were terrorists.
But the so-called no-fly lists have generated criticism. Many people have been mistakenly stopped, while others assert they were on the list in part because of their strong liberal politics.
In a well-publicized incident last year, some two dozen members of a group called Peace Action of Wisconsin, including a priest, a nun and high school and college students, were detained in Milwaukee en route to a "teach-in" and missed their flight.
In San Francisco, meanwhile, Rebecca Gordon and Janet Adams, two self-described peace activists who help run a publication called War Times that has been critical of the administration's terrorism policies, were detained on their way to Boston. An American Trans Air employee told them their names appeared on a no-fly list, according to the A.C.L.U. lawsuit, which includes both women as plaintiffs.
Officials insisted they were not seeking to single out legitimate political critics. Ms. Adams's name may have been similar to that of another person on the no-fly list, they said.
Ms. Adams said in an interview that "it strains my credulity" to think that her longtime role as a political advocate did not play a part in the incident. "It's bad enough that the government is stopping people in these vast quantities," she said. "But then to learn that you can't even find out why they did it is just an additional injury."
In its lawsuit, the civil liberties union said it had documented 339 cases since the Sept. 11 attacks in which people at San Francisco International Airport were stopped and questioned because they were thought to be on the no-fly list. While the group's investigation has focused on San Francisco because of complaints there, it said the situation there offers a window into what is happening at airports around the country, based on anecdotal evidence the group has collected.
"There's every reason to believe this is happening at airports around the country," said Jayashri Srikantiah, staff lawyer for the A.C.L.U. of Northern California.
The civil liberties union brought the lawsuit under the Privacy Act and the Freedom of Information Act after federal officials turned down several months of requests for information on the passenger lists.
The F.B.I. told the group in a letter last December that it found "no records pertinent" to the no-fly issue. But A.C.L.U. officials said records from the San Francisco airport showed that the F.B.I. was contacted about many of the airport detentions.
Officials at the Transportation Security Administration, named as a defendant in the suit, did not return calls seeking comment. Officials at the F.B.I., also named as a defendant, said they could not comment because the lawsuit was pending.
But a law enforcement official, who would speak only if not named, acknowledged that there was confusion in the public about how the no-fly lists were created and executed. The official said the F.B.I. provided intelligence on people suspected of links to terrorism, which was relayed to the transportation security agency. Transportation officials then provide airlines and airports with lists of people to look for at airports.
The security agency "needs to do a better job of explaining what this list is," the official said.
The official insisted that politics had nothing to do with who makes the list, saying that "people that are expressing their constitutional rights of free expression would not come to the attention of the F.B.I."
Copyright 2003 The New York Times Company