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 to respond to threats of nuclear terrorism, and as many as 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 police officer’s radioactivity sensor.

But the teams have not become complacent. If the federal government’s many layers of defense against nuclear smuggling break down, these unarmed weapons designers and physicists, along with experts from the FBI, are 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 Sept. 11, 2001, the office has created 26 rapid-response units. If a device is located, two other specialized teams would rush to the scene, one from a base in Albuquerque, N.M., 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, and log the evidence.

About 1,000 nuclear weapons scientists and 500 to 1,000 FBI professionals overall 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.

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 Association for the Advancement of Science and led by Stanford University physicist Michael M. May.

In the meantime, the U.S. is retrieving and locking down nuclear fuels abroad, has created a line of radiation detectors at foreign and domestic ports and has increased its intelligence efforts. If those and other measures fail, the emergency response teams are the last hope, but one nobody should rely upon, 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.

The full capability of the teams is classified. Bruce Goodwin, the nuclear weapons chief at Lawrence Livermore National Laboratory, noted in an interview that the teams now have “some really remarkable tools that can prevent nuclear function,” suggesting a device that can foil the arming system or perhaps even neutralize its basic operation.

It is assumed that any terrorist bomb would have booby traps and anti-tampering devices, perhaps designed by scientists who studied at the same universities that trained U.S. weapons scientists. Emergency response scientists run exercises in which one team designs a booby-trapped bomb and another team tries to disarm it.

A weapon stolen from a national stockpile might pose fewer problems than a makeshift terrorist device.

“We know a lot about other people’s weapons,” Curtis said. “They will tolerate a greater intrusive disarming strategy than an improvised nuclear device.”

History has some unfortunate lessons. In 1980, a team of Energy Department experts was sent to help disarm a large conventional bomb placed by an extortionist at Harvey’s Resort Hotel in Stateline, Nev. The bomb had extraordinary anti-tampering devices that prevented the teams from disassembling, disarming or even moving it.

So the bomb experts decided to fire a shaped-charge explosive into the arming mechanism, hoping to sever it from the rest of the bomb before it could detonate. After the hotel was evacuated, the team triggered the charge from a safe distance. The strategy failed, leaving the hotel extensively damaged when the 1,000-pound bomb blew up.

But today’s level of expertise would easily have solved the problem, said Joseph J. Krol Jr., a retired Navy rear admiral who heads the National Nuclear Security Administration’s Office of Emergency Operations, to which Wilber’s emergency response office belongs.

“We are very much better prepared,” Krol said. “How we operated then and how we operate now is like night and day.”

Indeed, Philip E. Coyle, a former deputy director at Lawrence Livermore, recalled that when he served on the emergency teams in the 1970s and 1980s, he carried a card to present at an airport in an emergency so he could order airlines to take him somewhere.

“It sounded good, but I always wondered whether it would work,” he said. Now the teams travel by government aircraft and other federal vehicles.

A successful terrorist nuclear attack would trigger a “national response plan.”

Many federal agencies would swing into action, including the Environmental Protection Agency, the Defense Department, the Department of Homeland Security, the Department of Justice, and a myriad of obscure offices.

For example, the National Atmospheric Release Advisory Center, based at the Livermore lab, would run advanced computation models of fallout patterns to provide evacuation plans for potentially millions of Americans.

Whether so many federal departments and all their agencies could work together in the chaos of a nuclear attack, as well as coordinating with state and local officials, is a matter of concern in Congress. But Majidi and Krol say extensive planning and practice exercises have clarified the lines of authority.

Communications would be a major undertaking.

“If you tell 100 million people to go east, 25 million will go west because they don’t trust the government,” said Jay C. Davis, a retired weapons scientist who is working on the nuclear forensics study.

The study is trying to assess how authoritatively the U.S. can attribute the source of a nuclear device and then make its case to the American public and the rest of the world.

Officials hope that nuclear forensics would allow scientists to assess the size of a detonation within one hour, the sophistication of the bomb design within six hours, how the fuel was enriched within 72 hours and the peculiar details of national design within a week, said Davis.

“Does this look like a Russian, a Chinese or a Pakistani device or something we have never seen before?” he said.

AP-NY-01-05-08 2323EST