The full-body scanners in use at 78 U.S. airports can detect small amounts of contraband and hidden weapons, all while producing controversial images of travelers.
The "good catches," federal officials say, have largely gone unnoticed amid the criticism that erupted over the ghostly X-rays and "enhanced" pat-downs.
The Transportation Security Administration, which intensified airport screening last month, points to several successes: small amounts of marijuana wrapped in baggies, other drugs stitched inside underwear, ceramic knives in shirt pockets.
But the machines could miss something far more deadly: explosive material taped to someone's abdomen or hidden inside a cavity. Researchers and security experts question the technology's ability to detect chemical explosives that are odorless and easily molded to fool machines and security screeners. Government testing has also raised concerns about the effectiveness of the full-body scanners.
Federal officials plan to continue an unprecedented rollout of the scanners over the next year. By New Year's Day, about 500 machines will be in use across the country. By the end of next year, 1,000 X-ray machines will be operational, accounting for roughly half of the nation's 2,000 lanes of security checkpoints.
Federal officials say the scanners represent the best technology that has passed both lab and field tests. "The bottom line is that we are now able to detect all types of the most dangerous weapons - nonmetallic explosive devices," TSA spokesman Nicholas Kimball said.
Two types of scanners, backscatter and millimeter wave, have been installed at airports since 2007. Both machines produce the same full-body images and both work by bouncing X-rays or radio waves off skin or concealed objects.
"It's not an explosives detector; it's an anomaly detector," said Clark Ervin, a former inspector general for the Department of Homeland Security. "Someone has to notice that there's something out of order."
Brian Michael Jenkins, of the Mineta Transportation Institute in San Jose, Calif., said he was unsure whether an advanced scanner would have detected PETN, or pentaerythritol tetranitrate, the odorless, malleable explosive used in last year's Christmas airliner bombing attempt. But a random assortment of security measures, not reliance on one technology or method, is key, he said.
The federal Transportation Security Laboratory in New Jersey began testing full-body scanners in 2007. Details are classified, but according to interviews and a limited release of lab findings:
Detection of weapons and contraband varied by who was evaluating the images.
Backscatter rays can be obscured by body parts and might not readily detect thin items seen "edge-on."
Objects hidden in body cavities might be missed by both types of the scanning machines.
To address the litany of security and privacy concerns over the full-body scanners, federal officials are testing several new technologies. But TSA Administrator John Pistole said that so far software patches for the scanners are producing too many false-positive errors in lab tests.