Civil liberties groups and privacy advocates are urging federal officials to slow down the rush to buy more full body airport scanners until there is more proof that the machines would disrupt a future terror attack.
Nineteen airports already have 40 of the machines in use, 150 more are scheduled to be installed this year, and the Transportation Security Administration recently announced it has secured funding for an additional 300.
At the same time, officials in Great Britain and the Netherlands who had been slow to introduce the machines now say passengers on transatlantic flights will be required to pass through them.
Mike German, a former FBI agent who now serves as policy counsel for the American Civil Liberties Union, said that while longstanding privacy concerns have not gone away, the real concern should be whether the machines will do anything to improve the safety of the traveling public.
"The question should be not whether it's worth the privacy invasion, but whether it's worth investing in technology that the terrorists are already showing can be defeated," German told ABC News. "There is no reason to sacrifice our liberty or privacy if there is no security gain."
German based his doubts about the technology's effectiveness on a report Sunday in the British newspaper The Independent, which cited 2005 tests there showing that the millimeter-wave scanners detected shrapnel and heavy wax and metal, but did not spot plastic, chemicals and liquids.
In the days since the Christmas bombing attempt on Northwest Flight 253, current and former officials in the U.S. and overseas have touted the machines as a potential new tool to thwart similar attackers.
On Sunday, former Homeland Security Secretary Michael Chertoff told NBC's David Gregory that he believed the scanners would have detected the explosives that Nigerian Umar Farouk Abdulmutallab allegedly carried onto the Detroit-bound flight.
"I believe the answer to that is yes. Of course, no technology is perfect, but this would dramatically increase the ability to detect things that are concealed underneath people's clothing, on their bodies," he said.
Chertoff disclosed that his consulting firm had been hired by one of the handful of companies that make the scanners.
Three of the manufacturers -- Rapiscan Systems, American Science Engineering, and L-3 Communications -- have lobbyists on Capitol Hill pushing back against concerns about the invasive aspects of the body imaging.
Last summer, the House passed language in a TSA spending bill aimed at limited use of the scanners. The language proposed that the machines be used not for primary screening of passengers, but only in cases when screeners believe an additional search is warranted. The Senate has yet to act on the measure.
Six of the machines, made by L-3 Communications, are being used for what TSA calls "primary screenings" at six U.S. airports: Albuquerque, Miami, Las Vegas, San Francisco; Salt Lake City, and Tulsa.
This means passengers go through the scans instead of a metal detector. They can opt for a pat-down search from a security officer instead.