Despite urgings from Congress and the Sept. 11 commission, U.S. commercial jets remain vulnerable to suicide bombers, aviation security experts said.

Fresh concerns arose about suicide bombings after Russian officials said Friday that at least one of the two planes that crashed simultaneously this week fell victim to a terrorist attack. Officials said they found traces of the explosive hexogen, also known as RDX or cyclonite, at one of the crash sites. The discovery raises the likelihood that explosives were brought onto the aircraft. Russian security officials are speculating that one Chechen woman with explosives boarded each plane.

U.S. security experts said the latest details seemed to point to the possibility of a suicide bombing -- a threat that airline security experts in the United States have long known they do not have sufficient means to prevent. One of the Sept. 11 commission's key recommendations for aviation security is to screen passengers, not just carry-on bags, at checkpoints for explosives.

"The TSA and the Congress must give priority attention to improving the ability of screening checkpoints to detect explosives on passengers," the Sept. 11 commission report said. "As a start, each individual selected for special screening should be screened for explosives."

The Transportation Security Administration said it is testing walk-through, explosives-detection portals at five airports this summer. The agency said it plans to buy 10 machines, which cost $150,000 each, according to one vendor.

"We will continue to aggressively test new technologies and pursue the possible expansion of the trace portal machine" program, said TSA spokeswoman Amy Von Walter.

Security experts said the portals, made by divisions of General Electric Co. and Smiths Group PLC, would be able to detect the hexogen explosive found at the Russian crash site, depending on how it was created. Hexogen, which the military has used since World War II, is a white crystalline substance that is often combined with other materials to make highly powerful explosives. When combined with plasticizers, hexogen is used to form a more sophisticated and difficult-to-detect explosive called C-4.

The explosives-detecting machines, which are larger and taller than a typical walk-through metal detector, puff air onto a person's body and then sniff for traces of explosives. The process takes about 15 seconds per passenger.

"The thing that's frustrating is that we have these devices manufactured by Smith and GE. They've been tested. They work. They're not particularly expensive," said Peter Goelz, former managing director for the National Transportation Safety Board whose firm does work with GE Security. "If this isn't a wake-up call, then shame on our security apparatus."

The walk-through portals, like any technology, have drawbacks. The machines could not be used on every airline passenger because they are too slow. They also may not be able to pick up certain plastic explosives when they are well-packaged and hidden, said John Huey, a security consultant who has tried to get the agency to adopt new technologies at the checkpoint.

"If a trace amount of RDX gets up to that detector, it will detect it," Huey said. But, he added, "None of these things can be relied upon 100 percent. You have to have redundant systems."

TSA said it is also interested in a technology called backscatter, which provides detailed outlines of a person's body and can see through clothing. The agency said it still wants to find a way to answer concerns about passengers' privacy before moving ahead with the technology.

The threat of suicide bombers strapping explosives to their bodies and hiding them under clothing has been well known. Richard Reid attempted to ignite his explosives-laden shoes while on an American Airlines flight from Paris to Miami in December 2001, and he was stopped only after boarding the plane by alarmed flight crew and passengers.

In 1994, Ramzi Ahmed Yousef brought explosives onto a Philippine jet and left them on board, killing a Japanese passenger on the next flight. Yousef, who is in U.S. custody, also plotted to blow up a dozen U.S. airliners over the Pacific Ocean the following year.

Last year, Department of Homeland Security officials alerted airports and security screeners that terrorists might use several different methods to sneak explosives through airport security checkpoints, including hiding them inside cell phones, cameras, stuffed animals and jackets.

In a hearing this week on the Sept. 11 commission's recommendations, Rep. Peter A. DeFazio (D-Ore.) asked the TSA why it had not moved more quickly to protect airlines from terrorists wearing suicide belts. He said the belts are "one of the most extraordinary points of vulnerability" in U.S. aviation.

Staff researcher Richard Drezen contributed to this report.