Updated

The American Civil Liberties Union announced Tuesday that it would file a class-action lawsuit challenging the list of travelers the government has barred from flying because they are considered threats. The civil rights group is representing seven plaintiffs.

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

"This can't be good for security because again the people who are targeted are innocent people," said Reginald Shuford, ACLU senior staff attorney. "For people who are legitimately involved in terrorist activity then by all means use the list to do what you need to do to protect all Americans and to guarantee safety, but these people are innocent people; they have nothing to do with terrorism whatsoever," he said.

Raw Data: Green v. TSA

The names on the no-fly list (search) are gathered from the FBI, CIA and other intelligence sources. Airlines compare the no-fly list with passenger manifests. If a name matches, the passenger is not allowed to fly until additional security checks are done — measures that usually involve an interview by the FBI.

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

The plaintiffs in this case allege that being on the list causes them repeated delays when boarding a flight, as well as the stigma of being singled out as a security threat in front of passengers and flight crews.

Little else is known about the lists, including how many people are on them and by what criteria a person is placed on the lists or removed.

"When the government starts singling you out for negative treatment, it is hard to know where that is going to stop," said ACLU attorney David Fahti. "Nevertheless, I continue flying, and I intend to do everthing to safeguard my rights and the rights of my fellow airline passengers," he said.

The TSA acknowledged the name-matching technology used by some airlines can confuse names 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, said Mark Hatfield, TSA spokesman.

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

"There doesn't seem to be any reliable way to resolve the problem that these people continuously confront," Sobel said. This is also a claim of the ACLU, which said it is virtually impossible to get off the list even when the FBI has done security checks to clear your identity.

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

A spokesman for the TSA admitted to Fox News that the no-fly list is not perfect—there are inconsistencies, and it's true that some people have a tough time getting off the list. That's why the TSA says it's pushing for a more comprehensive screening system, which uses addresses, social security numbers and dates of birth. It is called CAPPS II or the Computer-Assisted Passenger Prescreening System (search), but this system is also controversial.

Critics 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.

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.

Fox News' Catherine Herridge and the Associated Press contributed to this report.