"Screening shoes by X-ray is an effective method of identifying any type of anomaly, including explosives," Transportation Security Administration chief Kip Hawley said at a news conference Tuesday at Reagan National Airport just outside Washington.
A study by the Homeland Security Department, obtained by The Associated Press, states that X-ray images "do not provide the information necessary to effect detection of explosives."
Under new orders this week, all airline passengers must put their shoes through X-ray machines before boarding their flights.
There was mixed reaction to the mandatory order at Boston's Logan Airport Monday.
"If it means that people feel more secure, there's a way to have safety be priority, then I'll take my shoes off," one woman passenger told Kim Tunnicliffe of CBS Radio station WBZ.
"It's just inconvenient. It's just one more example of being inconvenienced by this whole war on terror," said another.
A scientist who has studied the issue said the truth lies somewhere between the study's findings that X-ray machines cannot detect bombs and Hawley's assertion that they can.
Richard Lanza, senior research scientist at the Massachusetts Institute of Technology, said the X-ray image does not identify what a gel or a liquid is made of. But, Lanza said, screeners can "look at the image and connect regions that look the same in density and shape."
It is not a foolproof method, but it is often effective, he said.
"Nothing is 100 percent," Lanza said. "But if the bad guys think you have a good shot at discovering it, they'll do something different."
Hawley said that 31,000 screeners have been specially trained to see if a shoe has been tampered with when they look at its X-ray image.
"It does take the human brain to make the interpretation on X-ray, but it is, frankly, not the most difficult thing we have to do to find potential shoe bombs," Hawley said.
He displayed copies of X-ray images of two pairs of shoes one with no explosive device and one worn by Richard Reid, who was arrested aboard a trans-Atlantic flight in 2001 when he tried to ignite a bomb in his shoe.
"You can see very clearly the difference between a shoe with an explosive and one without," Hawley said.
But the Homeland Security Department said in its April 2005 report that the ability of screeners to detect improvised explosive devices "is not a matter of proper training, reinforcement or motivation." The report is titled "Systems Engineering Study of Civil Aviation Security Phase I."
The report cited studies that show a person who has made or carried a bomb is likely to have traces of explosives residue on his hand. The report recommended that screeners use a technology called explosives trace detection, or ETD, on the shoes and hands of passengers who arouse suspicion or are randomly chosen for more screening.
ETD is commonly used at airports by TSA screeners, who use a dry pad on the end of a wand to wipe a surface baggage, shoes, clothing. They then put the pad into an ion mobility spectrometer that can detect traces of explosives.
The TSA's new screening procedures were ordered after British police last week broke up a terrorist plot to assemble and detonate bombs aboard as many as 10 trans-Atlantic flights from Britain.
Airline passengers can no longer carry liquids and gels into airline passenger cabins. Their carry-on luggage is searched by hand more, and they are subject to random double screening at boarding gates.
On Sunday, the TSA made it mandatory for shoes to be run through X-ray machines as passengers go through metal detectors. The checks were begun in late 2001, after Reid's arrest, and have been optional for several years.