The government sought to assure airline travelers Tuesday that X-raying shoes at security checkpoints was a reliable way of detecting weapons and explosive devices.
"Screening shoes by X-ray is an effective way of identifying any anomaly, including explosives," said Kip Hawley, Transportation Security Administration chief, at a news conference at Reagan National Airport just outside Washington.
Under new orders this week, all airline passengers must put their shoes through X-ray machines at checkpoints.
But according to a Homeland Security report on aviation screening obtained by The Associated Press, the machines don't help screeners find a specific liquid or gel that can be used as a bomb.
At the news conference, Hawley said the machines can detect improvised explosive devices, which he said were "the No. 1 threat that we guard against."
He displayed a mock-up of shoes worn by Richard Reid - arrested aboard a trans-Atlantic flight in 2001 when he tried to ignite an explosive device hidden in his shoe - and shoes with no explosive device.
"You can see very clearly the difference between a shoe with an explosive and one without," Hawley said.
The new screening procedures were put in place after British police last week broke up a terrorist plot to assemble and detonate bombs aboard as many as 10 airliners crossing the Atlantic Ocean from Britain to the U.S.
Among the new procedures are a ban on liquids and gels in airline passenger cabins, more hand searches of carryon luggage, and random double screening of passengers 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. They were begun in late 2001, after the Reid's arrest. The shoe scans have been optional for several years.
In its April 2005 report, "Systems Engineering Study of Civil Aviation Security - Phase I," the Homeland Security Department concluded that images on X-ray machines don't provide the information necessary to detect explosives.
Machines used at most airports to scan hand-held luggage, purses, briefcases and shoes have not been upgraded to detect explosives since the report was issued.
TSA contends, however, that screening shoes is an important security strategy for detecting concealed weapons or tampering.
"Our security officers, after they've screened thousands of shoes, can see that shoes have been tampered with or an anomaly in the shoe," said TSA spokeswoman Ellen Howe.
She also said that TSA doesn't need large bomb-screening equipment to find a problem in a shoe. "We've definitely found things that need to looked at further," she said.
The Homeland Security report said that "even a 1/4-inch insole of sheet explosive" could create the kind of blast that reportedly brought down Pan Am flight 103, the airliner that blew up over Lockerbie, Scotland, in December 1988, killing 270 people in the air and on the ground.
The Homeland Security report recommends that explosives trace detection, or ETD, be used on the shoes and hands of passengers when the screeners determine they must be checked more thoroughly.
"To help close this gap, the percentage of shoes subjected to explosives inspection should be significantly increased," the report said.
"Within the current state of the art, they afford the only meaningful explosives detection capability at the checkpoint," the report said.
ETD involves a screener using a dry pad on the end of a wand to wipe a surface - baggage, shoes, clothing - and then putting the pad into a machine called an ion mobility spectrometer. The machine can detect tiny particles, or traces, of explosives.
Screeners do use ETD on passengers who have been selected to be screened a second time after going through the checkpoint.
The agency is testing equipment to detect liquid explosives at six airports, Hawley said, and he called the technology "very promising."
But, he said, "with a million and a half to 2 million passengers every day, it is not practical to think that we are going to take every bottle and scan it through these liquid scanners."
The government's new order that all airline passengers put their shoes through X-ray machines won't help screeners find a liquid or gel that can be used as a bomb.
X-Ray machines are unable to detect explosives, according to a Homeland Security report on aviation screening.
The TSA ordered the shoe-scanning requirement as it fine-tunes new security procedures.
X-ray devices show carry-ons in 3-D