American Civil Liberties Union's officials declined to comment in advance of their planned announcement Tuesday that they would file a class-action lawsuit challenging the list of travelers that the government has barred from flying because they're considered a threat. The civil rights group is representing seven plaintiffs.

Airlines are instructed to stop anyone on the "no fly" list that is compiled by the Transportation Security Administration (search). The ACLU (search) contends, though, that some people are wrongfully put on the list.

"Many innocent travelers who pose no safety risk whatsoever are stopped and searched repeatedly," the ACLU said in a statement issued Monday.

The no-fly list is one of two lists kept by the TSA. The other is the "selectee." Those on the no-fly list are not allowed to board a commercial aircraft. Those on the selectee list must go through more extensive screening before boarding.

Federal law enforcement and intelligence agencies request that the TSA put names on the list.

Little else is known about the lists, including how many people are on them and how they qualify to get on or off.

The TSA acknowledged the name-matching technology used by some airlines confuses people on the no-fly list with passengers who have similar names.

In such a case, a passenger would be referred to a law enforcement official, who would be able to clear up mistakes by checking the person's identification and perhaps putting in a call to the FBI (search), Mark Hatfield said.

Problems with the no-fly list have provided fodder for critics of the TSA plan to conduct computerized background checks of all airline passengers and to rank them according to their risk of being a terrorist. They say that if a no-fly list with relatively few names causes confusion and produces misidentifications, the government cannot be trusted with a far broader program.

Some people on the no-fly list have found it impossible to get off, said David Sobel, general counsel for the Electronic Privacy Information Center.

"There doesn't seem to be any reliable way to resolve the problem that these people continuously confront," Sobel said.

Hatfield acknowledge such problems exist but said the agency has worked to help people wrongly identified.

Separately, the TSA said Monday that it is seeking proposals from companies to run a pilot "registered traveler" program in which low-risk frequent fliers could avoid extra security inspections at airports by submitting to background checks.

Companies are being asked to show how they'd manage the program, as well as their capabilities in biometric identification and computer technology.