Passengers identified for additional security screening at Reagan National Airport will now have their boarding passes checked for explosives using document-scanning machines installed yesterday at one of the airport's checkpoints.

The Transportation Security Administration said National is the first of four airports to begin testing the machines over the next 30 days. The machines can detect whether a passenger has recently handled explosive materials by analyzing residue passed from the hands onto the boarding pass.

The document-scanning effort is the latest in a series of pilot projects involving explosive-detection systems that the TSA has undertaken this summer. The moves have come in response to the 9/11 Commission's recommendations and criticism from members of Congress that U.S. airports have been slow to provide defenses against a suicide bombing attack.

Explosive residue was found at the crash sites of two Russian planes that went down nearly simultaneously last month, although officials have not determined how the explosives got on board. TSA spokeswoman Amy Von Walter said the tests were not being conducted in response to the crashes, but that the agency is looking more aggressively for new technologies to prevent such attacks. The agency uses explosive-detection machines to scan checked luggage but does not regularly check passengers or carry-on luggage for explosives at security checkpoints, as the 9/11 panel recommended.

"The TSA's attention has always been to leverage new and existing technologies across all modes of transportation," Von Walter said.

This summer, the TSA began testing walk-through portals at five airports that can detect explosive residue on clothes by puffing air at them and "sniffing" for the residue. The program will be expanded to 10 additional airports next year. Also, at six airports, the agency began a test program to screen cargo for explosives.

The document scanners, sold by Smiths Detection, a unit of Smiths Group PLC of London, are stationed beyond the metal detectors at National's southern security gates serving American and Alaska airlines. If passengers set off the metal detector or if they have been preselected for additional scrutiny, a security screener will rub both sides of the passenger's boarding pass across a small piece of cotton gauze attached to the machine.

The screener then pushes a button and the gauze slides into the machine to be analyzed. Within seconds, a small color screen displays whether the material contains traces of explosives. The machines cost $40,000 to $50,000 each, according to a Smiths Detection spokesman.

In a demonstration yesterday, National's security director, Patrick D. Hynes, said if a passenger had been handling explosives and touched a boarding pass, the machine would pick up "even the most minute particle of explosive material."

"We are privileged at Ronald Reagan National Airport to be the first airport to have this technology," Hynes said.