No practical technology exists to detect someone carrying explosives onto a subway or a bus the way four men did in London 12 days ago, federal authorities said yesterday.

The most effective method for finding explosives in a backpack or on a person boarding a subway or bus remains the use of dogs trained to sniff explosives, said officials from the Bureau of Alcohol, Tobacco, Firearms and Explosives.

Bomb-sniffing dogs are used by a number of agencies, but there are only about 100 ATF-trained "explosives-detection canines" nationwide. Experts also said the bomb-sniffing dogs are limited in their abilities by a range of factors, including the strength of the explosive's odor and how far away the dogs are from a person carrying a bomb.

Private companies, government agencies and scientists at U.S. laboratories and defense research centers are working to develop technologies that could possibly be used on mass-transit systems that carry 14 million people to work every day.

"If there was anything else out there now, we would be screaming that everyone on Metro should walk through a detector," ATF spokesman Richard Marianos said. "It just is not there."

Washington has received, along with New York City, the most federal money for its transit system -- more than $49 million since Sept. 11, 2001 -- for cameras, canine units and other equipment designed to "harden" the system and discourage a terrorist attack.

After the London bombings, transit officials in Washington stepped up security using dogs, cameras and police toting automatic weapons. But even its most sophisticated equipment -- the PROTECT system of chemical sensors installed at half of Metro's underground stations -- is not designed to prevent an attack but rather to minimize casualties and reduce the impact of a chemical release.

"How can you possibly sniff out everyone carrying explosives?" asked Fred Goodine, Metro's assistant general manager for system safety and risk protection. "The technology isn't there, at least today. Not if you want it to be an open system, which is what mass transit is."

After the terrorist bombings of commuter trains in Madrid that killed nearly 200 people in 2004, officials at the Department of Homeland Security began an experiment at a Maryland train station to explore whether it was feasible to screen rail passengers with bomb-detecting equipment.

During the 30-day, $1 million pilot project in May at the New Carrollton Amtrak/MARC station, riders had to walk through a high-tech "sniffer," developed by General Electric Infrastructure Security, that checked them for bomb residue.

Passengers had to pause in a security portal for 12 seconds while a sensor in the ceiling "sniffed" for traces of explosives. The equipment shot eight puffs of air at the passengers' upper thighs to help free any particles that may have been clinging to clothing.

Transit officials said the system was too time-consuming and trains were delayed. It has not been installed in any mass-transit systems, where the high volume of riders and trains spaced just minutes apart make the screenings too difficult.

The bomb-sniffing device "doesn't practically fit into the open infrastructure of mass transit," said Greg Hull, director of safety and security programs at the American Public Transportation Association, who worked on the New Carrollton project.

The "puffer machines" are being used, however, at about 16 airports nationwide -- along with X-ray machines that scan luggage and a trace-detection machine that uses a cotton swab to test for residue, a spokesman for the Transportation Security Administration said.

Police used portable trace-detection machines at two subway stations in Boston during last summer's Democratic National Convention. But officials said they worked only because passengers were understanding about the security delay and they would not be feasible during normal operations.

Although a CNN-Gallup poll showed that 69 percent of Americans favor "requiring every American to go through a metal detector when using public transportation, including trains, buses or subways," Homeland Security Secretary Michael Chertoff said authorities will never install metal detectors in the nation's subways because the volume of passengers is too great.

"Can you envision magnetometers on the New York subway?" he asked during an interview at The Washington Post last week. "If the subway doesn't work because of the security measures, then we have lost the war, because then they have driven us out of the subway."

Chertoff said some train stations now have devices that can detect certain biological agents in the air, "but there is no single system that exists that allows us to guarantee people are not going to get on a train with explosives."

Joseph M. Riehl, the chief of the ATF arsons and explosives programs division, said that about 100 dogs have been trained at Front Royal, Va., to detect about 19,000 types of explosives. An additional 400 dogs are being used across the world, including one dog who is working to detect explosives in Iraq.

Bomb-detecting dogs can "alert" on firearms, explosives and ammunition hidden in containers and vehicles, on people and buried underground, Riehl said.

"There is nothing we have identified at this point that would work any better than the dogs," he said.

Researcher Julie Tate and staff writer Sara Kehaulani Goo contributed to this report.