For the last few years, federal agencies have defended body scanning by insisting that all images will be discarded as soon as they're viewed. The Transportation Security Administration claimed last summer, for instance, that "scanned images cannot be stored or recorded."

Now it turns out that some police agencies are storing the controversial images after all. The U.S. Marshals Service admitted this week that it had surreptitiously saved tens of thousands of images recorded with a millimeter wave system at the security checkpoint of a single Florida courthouse.

This follows an earlier disclosure (PDF) by the TSA that it requires all airport body scanners it purchases to be able to store and transmit images for "testing, training, and evaluation purposes." The agency says, however, that those capabilities are not normally activated when the devices are installed at airports.

Body scanners penetrate clothing to provide a highly detailed image so accurate that critics have likened it to a virtual strip search. Technologies vary, with millimeter wave systems capturing fuzzier images, and backscatter X-ray machines able to show precise anatomical detail. The U.S. government likes the idea because body scanners can detect concealed weapons better than traditional magnetometers.

This privacy debate, which has been simmering since the days of the Bush administration, came to a boil two weeks ago when Homeland Security Secretary Janet Napolitano announced that scanners would soon appear at virtually every major airport. The updated list includes airports in New York City, Dallas, Washington, Miami, San Francisco, Seattle, and Philadelphia.