(The following article by Eric Lipton and Andrew C. Revkin was posted on the New York Times website on July 11.)
WASHINGTON — With the mass transit systems in Britain and the United States on high alert, the best available defense the governments can provide against a terrorist armed with a bomb is decidedly low-tech: vigilance with dogs, video cameras and security officers.
That limited arsenal has provoked calls for an accelerated campaign to develop high-tech tools like artificial noses that sniff out explosives or devices that can detect bombs through clothing.
“We need a crash program,” said Senator Charles E. Schumer, a Democrat from New York. “Terrorists have chosen mass transit as their target of choice.”
But installing such technologies would take years, cost billions of dollars and could prove impractical in dealing with millions of passengers every day, transit officials and security experts say. And even then, they might not deter an attack.
“You can’t catch everything with gizmos, because the terrorists come up with some way to evade the gizmo,” said Jessica Stern, a terrorism expert at the Belfer Center for Science and International Affairs at Harvard.
At least for now, security experts said, the best options may be enhanced surveillance of suspected terrorist groups and specialized training for transit personnel in skills, developed in Israel, to identify a terrorist before he or she strikes.
“The best-case situation is for us to intercept an attack before it happens, having the intelligence capability to learn if something is happening,” said Paul J. Browne, a spokesman for the New York Police Department.
The push to develop new methods of transit security in the United States started in earnest after the Madrid train bombings in March 2004. The federal government set up more canine bomb-detecting teams, and the Department of Homeland Security required transit systems to remove trash cans from certain areas, inspect more frequently for suspicious packages and urge passengers to report unattended bags.
Security experts are also urging transit systems to take measures to limit casualties in the event of a successful attack. These include installing windows in subway cars that blow out to reduce the destructive force of a blast and improving ventilation systems to expel deadly smoke and fumes after an explosion or fire.
Rafi Ron, a former security director at Ben-Gurion International Airport in Israel, said the next step was to train transit officers and subway and bus operators to help identify likely terrorists. Bus drivers in Israel and police officers at Logan International Airport in Boston, for example, already are encouraged to examine how passengers behave, including how they dress and walk.
“It is not a foolproof solution,” Mr. Ron said. “But there have been instances in Israel where bus drivers, as well as security personnel, have detected terrorists early enough to prevent them from reaching the target.”
Still, training transit workers to be watchful, while helpful, “is not sufficient,” said Brian M. Jenkins, the director of the National Transportation Security Center of the Mineta Transportation Institute, a research organization in San Jose, Calif.
“London is a model system,” he said, with decades of practice against bombing campaigns of the Irish Republican Army. “But despite that fact, these four incidents occurred,” he said, referring to the subway and bus bombings on Thursday.
New technologies could play an important role in detecting weapons, said Donald W. Tighe, a spokesman for the Science and Technology Directorate at the Department of Homeland Security.
The department has given research grants to companies to explore computer programs that take closed-circuit television systems and automatically identify suspicious activity – like wearing a coat in warm weather. Advanced cameras and computers might also be able to check images against a database of known or suspected terrorists.
Homeland Security officials are also evaluating 40 applications from companies seeking grants – worth up to $9 million in all – for the rapid development of devices that can find bombs at distances much greater than a specially trained dog can.
The goal, the homeland security secretary, Michael Chertoff, said Sunday, is to figure out a way to create a device that can quickly or even simultaneously check large numbers of people in a transit system, revolutionizing passenger screening.
“I can envision technologies in the future that will give us the ability to detect explosives without going through the kind of portals you do in the airport,” Mr. Chertoff said in an appearance on the NBC News program “Meet the Press.” “There is a technological and a system-based solution for this.”
Millimeter wave machines, one of the options under consideration, rely on a type of energy that uses trace amounts of heat released by objects, including humans, to create images that can identify hidden bombs or weapons from about 30 feet away.
Terahertz radiation devices can create images of concealed objects as well as identify the elemental components of a hidden item.
But both approaches face significant obstacles. The millimeter wave machines, as now designed, would require a screener to evaluate images of individuals, which would be impractical given the thousands of people who enter a single subway station. The devices would probably produce many false alarms.
The terahertz devices may be more promising since they could sound an alarm if someone entering a subway or train station had traces of elements used in bombs on them. But such machines are years away from being ready for use, one Homeland Security official said.
“It is way, far out on the horizon,” said Roshni Sherbondy, an explosives expert at the agency.
Less complex options – like a device that sniffs the air for traces of explosives as a person passes by or a metal detector that would look only for large volumes of metal that might be used to fill a bomb – also have their limits.
Such a trace detection device, which the federal government is now installing at airports, can handle only about seven people a minute, far too few for a mass transit system. And the metal detectors, while they could be set so they do not sound an alarm for routine devices like cellphones or belt buckles, would miss bombs that do not include metal.
Even if walk-through bomb-detection portals were installed at every entrance to all 468 New York City subway stations or all 2,000 Chicago bus stops, they would be useless unless security workers were there, ready to detain someone who set off an alarm, Mr. Jenkins said. A bomb might be set off in a subway or train station entrance, just beyond where the equipment would be set up.
Mr. Tighe said installing detection equipment at every subway station might indeed be impractical. But the technology might still be worthwhile if it could be used at select stations during times of heightened alert – like now – or after intelligence efforts turned up threats.
Mr. Chertoff said Sunday that he recognized the bottom-line challenge was figuring out a way to balance security and freedom of movement.
“You don’t want to put measures into effect in mass transit that defeat the purpose of mass transit, that prevent people from taking subways at all, because then we actually lose the war,” he said.
Ultimately, security experts and transit officials agreed, the best solution will probably be some combination of old and new technologies. Even then, given the tens of thousands of subway and bus stops across the United States and 30 million riders every workday, transit systems will remain a vulnerable and attractive target.
Eric Lipton reported from Washington for this article, and Andrew C. Revkin from New York. John Schwartz contributed reporting from New York.