Alexandra Hay is a 21-year-old college student, but when she tried to board a plane headed home for Thanksgiving, she was flagged as a potential terrorist and detained before eventually being released and allowed to fly home.

"I had no idea why I'd be on this list, or why anyone would want me on this list. It seems like a big mistake," she told reporters before suing the Department of Homeland Security, which, in turn, acknowledged that the appearance of her name on a “no-fly list” was indeed a mistake. The department said it would take immediate action to remove it.

Critics say chances of this kind of erroneous tagging of innocent Americans will only increase as the administration plans to roll out an even more ambitious screening program — the Computer Assisted Passenger Screening System (search) (CAPPS) II.

According to officials from the Transportation Security Administration (search), the new system would require airlines to check passengers' personal information at the point of ticket sale against a massive database that would include criminal and commercial records, and presumably a comprehensive Terrorist Screening Database (search) that is still being constructed by the Department of Homeland Security.

Government officials have said that credit and medical records will be off-limits.

Each passenger booking a flight will then be assigned a color code to determine how he or she will be vetted during check-in — green for standard screening, yellow for additional scrutiny and red barring the traveler from flight.

Officials from TSA, which is part of the Department of Homeland Security, say the new program will reduce the number of false positives and allow airlines to better screen for potential terrorists in a post-Sept. 11 world.

That’s what the Air Transport Association (search), which represents the major airlines, is hoping for. According to ATA spokesman Doug Wills, if properly implemented, the new system would actually reduce the chances that someone like Hay would be hauled off and questioned, the most problematic pitfall of the current screening system.

“CAPPS II has a lot of promise in that it may shorten a lot of airport lines, reduce traveler hassle in airports and reduce the number of people who will be pulled aside for secondary screening,” Wills said.

The government has been planning for CAPPS II for at least two years. Right now, the airlines are using a government-run CAPPS I (search) to screen passengers. Put into place before the Sept. 11, 2001, terror attacks to cut down on the potential for hijackings, the program checks passenger names against a list of terror suspects and flags travelers with particular characteristics, like buying one-way tickets with cash.

Despite the nearly $4 billion burden the system and other post-Sept. 11 security measures have placed on the airlines, Wills said the current screening process, which includes CAPPS I, has not proven an altogether efficient way to screen passengers. “I think everyone in the industry understands this,” he said, noting that the more rigorous screening is intended to fix that.

But critics say that CAPPS II will do nothing to ensure that innocent people won't get caught in a massive dragnet, while real terrorists will always find a way to exploit loopholes.

"Identity thievery provides such an easy method of bypassing such a profiling system," said Edward Hasbrouck, investigative reporter and author of "The Practical Nomad" travel series. "Without some way of preventing that, [CAPPS II] would be completely useless."

Hasbrouck and other privacy advocates say TSA is clearing the way for the government to collect and maintain massive dossiers on law-abiding citizens with no promises that the information won't be swapped on the open market and abused.

"It will be a very, very invasive system and I think it will be fraught with problems," said former Rep. Bob Barr, R-Ga. Barr said he has met with TSA officials regarding CAPPS II and it has done nothing to allay his fears.

"I have yet to be convinced that profiling law-abiding citizens will catch any terrorists," he told "They ought to be spending their time and money on terrorists and associates of known terrorists and running their names through a better terrorist watch list."

When CAPPS II was announced last year, several members of Congress expressed concerns over privacy, and said it would not fund CAPPS II until the General Accounting Office (search) conducted a study of the program. The study is slated for release Feb. 13, according to GAO officials.

But the government had already been testing the program and Homeland Security Undersecretary Asa Hutchinson said Monday that TSA would deploy it this summer.

According to confirmed reports, both Northwest Airlines (search) and JetBlue Airways (search) already have secretly handed over the personal data of 15 million passengers to government agencies and military contractors testing the utility of data-mining for airport screening. Hutchinson said the government plans to order airlines to provide background information on their customers to test the program.

"It certainly raises the question of whether other airlines have done the same thing and merely swept it under the rug," said Marcia Hofmann of the Electronic Privacy Information Center (search), which uncovered the information transfer between Northwest Airlines and the government, and is suing Northwest for violating its customer privacy policy.

Meanwhile, the major airlines say they will support the program if it has strong privacy controls for travelers, including a promise that personal information won't be handled by or shared with a third party.

“The government needs to address certain privacy concerns that the industry has about CAPPS II before it implements it,” said Wills, noting that the heads of the major airlines met with Department of Homeland Security officials for the first time on the issue Jan. 24.

Hasbrouck said many unanswered questions remain about the system's application: What commercial information will be used? If travelers don't have a phone number or a permanent address, will they not be allowed to travel? What about travelers who legally change their names — how will they be tracked?

"We don't know what is going to happen and to what degree it will impede people from flying or just be a nuisance," said Hasbrouck.

Representatives from the airlines said they have not yet worked out how they will implement privacy controls, and are still in discussions with the federal government on the details.

Chuck Pena, foreign policy expert with the Cato Institute (search), agreed that the government needs a better system for tracking and screening for terrorists, and CAPPS II could accomplish that. But, he added, the “utility of using commercial databases is next to zero."

“The more we do that,” said Pena, “the more the possibility for mistakes and flagging the wrong people."

And not everyone is willing to write off the program's utility yet. 

"I do think we need to be careful and review for any kinds of abuses," said Phil Kent, director of the American Immigration Control Foundation. "But I think when it comes to air travel, Americans want to feel safer. "

The Associated Press contributed to this report.