About every three days, unknown to most Americans, an elite team of federal scientists hits the streets in the fight against nuclear terrorism.
The deployments are part of an effort since 2001 to ratchet up the nation's defenses. More than two dozen specialized teams have been positioned across the nation to respond to threats of nuclear terrorism, and as many 2,000 scientists and bomb experts participate in the effort. Spending on the program has more than doubled since it was launched.
And an evolving national policy aims to create a system of nuclear forensics, in which scientific analysis could quickly identify the source of a nuclear attack or attempted attack. A key report on nuclear forensics is due next month.
The counter-terrorism efforts are becoming routine. Scientists in specially equipped helicopters and airplanes use radiation detectors to scan cities for signs of weapons. They blend into crowds at major sporting events, wearing backpacks containing instruments that can identify plutonium or highly enriched uranium.
So far, they have not encountered a terrorist. Near the Las Vegas Strip, they investigated a homeless person who somehow had picked up a piece of radioactive material. On the streets of Manhattan, a hot-dog vendor fresh from a medical test triggered a policeman's radioactivity sensor.
But the teams have not become complacent. If the many layers of federal defense against nuclear smuggling break down, these unarmed weapons designers and physicists, along with experts from the FBI, could be the last hope of staving off a catastrophic attack.
They are supposed to rush up to a ticking nuclear explosive (or a "dirty" bomb, which would disperse radioactive material) and defuse it before it's too late -- a situation often depicted by Hollywood that seems less fictional every year.
"After everything else fails, we come in," said Deborah A. Wilber, the scientist who directs the Office of Emergency Response at the Energy Department's National Nuclear Security Administration. "I don't believe it is a question of if it will happen. It is a question of when."
Since the attacks of 2001, the office has created 26 rapid-response units around the nation.
If a device were located, two other specialized teams would rush to the scene, one from a base in Albuquerque, where a fueled jetliner is on 24-hour alert. Another FBI team would depart from rural Virginia.
The teams would first attempt to disable a bomb's electrical firing system and then quickly transfer the weapon to the Nevada desert. There, the bomb would be lowered into the G Tunnel, a 5,000-foot-deep shaft, where a crew of scientists and FBI agents would attempt to disassemble the device behind steel blast doors, logging any evidence.
About 1,000 nuclear weapons scientists and 500 to 1,000 more FBI professionals participate in the nation's emergency response effort, though not full time. Increased investment in the project reflects an acknowledgment that the nation remains vulnerable to nuclear terrorism.
But the effort is also reaching for something greater than defense: a Cold War style of deterrence.
The scientists are also experts in the rapidly evolving field of nuclear forensics, which aims to track nuclear materials to their country of origin. Even if a bomb detonates, fallout can be analyzed to identify the terrorists and their state sponsors. A retaliatory strike could be the response.
The idea is to force other nations to take better care of their own nuclear fuels or else find themselves in the cross hairs of the U.S. nuclear arsenal.
A major technical and policy analysis of this approach -- the report that is due next month -- is being conducted by some of the nation's top nuclear weapons experts, sponsored by the American Assn. for the Advancement of Science and led by Stanford University physicist Michael M. May.
In the meantime, the United States is retrieving and locking down nuclear fuels abroad, has created a line of radiation detectors at foreign and domestic ports, and has increased intelligence efforts.
If those and other measures fail, the emergency response teams are a last hope, but one nobody should rely on, said Charles B. Curtis, president of the Nuclear Threat Initiative, which pushes for stronger efforts to prevent nuclear terrorism.
Intercepting a device "is a very, very, very difficult problem, but not impossible," said Curtis, a former Energy Department deputy secretary.
Vahid Majidi, a nuclear weapons chemist and head of the FBI Weapons of Mass Destruction Directorate, seemed more confident. Asked how good his chances would be to find a nuclear bomb in Manhattan with 24 hours' warning, he said, "Quite reasonable."