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 concealed 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, far smaller than previous incarnations, and easily molded to fool machines and security screeners into thinking they are part of the human body.

Government testing, which has been mostly classified because of security concerns, has also raised concerns about the effectiveness of the full-body scanners.

Based partly on early successes, federal officials are planning to continue an unprecedented roll-out of the technology over the next year. By New Year's Day, about 500 machines will be in use across the country, including at the Washington area's three major airports. 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.

Following the United States' lead, several nations have begun to test or install full-body scanners, including Australia, Canada, France, Germany, Japan, Russia and the United Kingdom. U.S. officials have also considered whether the machines could be used to enhance security at passenger rail stations.

Federal officials say the scanners represent the best technology that has passed both lab and field tests. But as with reading an X-ray, training is the most important factor in making sure that TSA officers can spot potentially dangerous items on passengers.

"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. "Even in small amounts, it can be picked up."

Window dressing?



Two types of scanning machines - backscatter and millimeter wave - have been installed at airports since 2007, when they were launched as part of a pilot program at Phoenix Sky Harbor International Airport. Both machines produce the same full-body images that attracted controversy; they work by bouncing X-rays or radio waves off skin or concealed objects.

They have been installed at a quicker rate since a failed Christmas Day terrorist attempt last year in which Umar Farouk Abdulmutallab hid explosives in his underwear on a flight from Amsterdam to Detroit. The failed attack also prompted federal officials to use the scanners as a primary security technique at airports instead of a secondary, less frequent checkpoint feature.

Still, many security experts say the machines are expensive window dressing meant to put the traveling public at ease.

A recent paper published in the Journal of Transportation Security by two former University of California-San Francisco physicists said that images produced by the backscatter scanners would probably fail to show a large pancake-shaped object taped to the abdomen because it would be "easily confused with normal anatomy." As a result, a third of a kilo of PETN, a type of malleable explosive, which could be discovered by a pat-down, would be missed, the scientists said.