Last year, as New Year’s Eve celebrations approached, clandestine teams of government experts took to the streets of four American cities, toting what appeared to be ordinary briefcases and golf bags. With the nation on high alert for terrorist attacks, the nuclear scientists bearing sophisticated radiation detection equipment scoured Washington, New York, Las Vegas and Los Angeles for “dirty bombs” – conventional explosives designed to spread radioactive material across a small area.

Starting Dec. 22, the squads from the National Nuclear Security Administration, part of the Department of Energy, fanned out across the four cities, taking measurements around the clock.

Their only radiation “spike” came from a rented storage facility near downtown Las Vegas. They found only a homeless man and the cigar-sized radium pellet, a medical device used to treat cancer, that he had found three years earlier and squirreled away in his pillow.

Government and independent analysts fear the day is coming, however, when a real dirty bomb or even a nuclear weapon will be there. Scientists from a cluster of programs within the NNSA designed to respond to a radiological emergency could be the best defense. Yet experts say even they may be unable to find the device.

“You have to have very sensitive instrumentation, and you have to be essentially right on top of it if they shield it pretty well,” Duane Sewell, 85, a former assistant energy secretary, said in a recent telephone interview. “And that doesn’t give you a warm, fuzzy feeling about being able to find stuff prior to some act that is going to let it loose in the atmosphere.”

Sewell helped create the Nuclear Emergency Support Team, a program within the NNSA comprising hundreds of federal nuclear scientists and technicians ready to rush to any U.S. city in search of these weapons. The experts called out in December were informally linked with NEST, for years a key federal bulwark against nuclear terrorism.

Supporters of the teams, and some independent analysts, say all is not well with NEST and its affiliated programs. Aircraft and other equipment are degrading, according to people familiar with the program. And as long ago as 1996, experts warned DOE of a growing talent shortage as nuclear scientists retired amid the downsizing of the nuclear weapons program after the Cold War.

Last year, DOE Inspector General Gregory Friedman warned in a report that NNSA’s top aircraft sometimes were unavailable to carry out missions and that contingency plans were lacking. Consequently officials “may not be able to respond as rapidly and effectively as necessary to address a potential terrorist incident,” Friedman said.

Another person familiar with the emergency response programs said, “Without trained, equipped experts in the field, no level of funding will save lives in a radiological dispersal event.”

Other experts say the government has overemphasized preventing a dirty bomb from going off, without adequately preparing for what to do if one does.

“It’s probably largely a waste of money, unless they have good intelligence on a specific scenario, to send people out in Times Square and so forth, looking for radioactive emissions,” said Thomas Cochran, director of the nuclear program at the advocacy group Natural Resources Defense Council.

Federal officials say the programs are properly focused and equipped to handle any emergency. An “equipment modernization program” underway has resulted in the retirement or replacement of some aircraft, and the updating of older custom-built detection equipment with newer, commercial units.

“Yes, the equipment is aging, a natural process,” NNSA spokesman Bryan Wilkes said in e-mailed response to questions. “However, over the last 2 years NNSA has invested in replacing critical equipment, and has added new equipment to our pool. … More equipment is scheduled for delivery this year, with more planned for the next five years.”

NNSA spokesman Kevin Rohrer acknowledged the administration is grappling with the post-Cold War brain drain. “There are some concerns out there that as the workforce ages and people retire, we’re losing some critical expertise,” he said in an interview. “And, on the other hand, we continue to try and hire and retrain people to fill those voids.”

NEST team members work for the Energy Department’s weapons plants and nuclear labs and related private contractors. Their volunteer duty keeps them on call for a week at a time, bags and bomb detection equipment packed.

NEST, based in Las Vegas, is the best-known but most secretive of seven related programs within NNSA, officials say. Other emergency response teams perform functions such as conducting air sampling from helicopters and fixed-wing aircraft, responding to nuclear weapons accidents, and monitoring radiation and public health in the event of nuclear accidents. The NNSA budget for emergency operations this year is about $89.7 million, including about $50 million for NEST.

The system of rapid-response teams of radiation experts dates to the Cold War, when such scientists were needed to help cope with accidents involving the nation’s nuclear weapons stockpile.

Air Force bombers were then in the air at all times to deter or respond to a Soviet nuclear attack. On Jan. 17, 1966, a B-52 bomber carrying four hydrogen bombs collided with a refueling plane and crashed near Palomares, Spain. Plutonium was scattered across more than 500 acres. It took the U.S. military nearly three months to decontaminate the area.

A staffer dispatched to Spain by Sewell, then deputy director at Lawrence Livermore National Laboratory in California, concluded that officials should be better prepared if there were another crash. The lab developed devices to detect gamma rays from downed weapons, and created “laboratories on wheels” that could be flown to accident sites, Sewell said.

When another B-52 went down two years later near Thule, Greenland, a Livermore team went to work.

Soon a new nuclear threat arose. In 1974, the FBI received a note saying that a bomb hidden in Boston would explode unless authorities paid a $200,000 ransom. It was a hoax, but the lumbering official response, plagued by travel mix-ups and logistical problems with equipment, exposed the government’s shortcomings in coping.

On Nov. 18, 1974, a secret Atomic Energy Commission memo to Mahlon “Ink” Gates directed the manager of DOE’s Nevada Test Site to take charge of a new field operation using radiation detection systems. The NEST program became official the following year.

Officials will not disclose how many times radiological emergency response scientists have been deployed or what, if anything, they’ve found on such missions. But news accounts suggest NEST teams were called out dozens of times through the 1980s and early 1990s.

About 12 years ago, DOE officials, guided by intelligence reports, began to alter training exercises to cope with the emerging threat of nuclear terrorism. Unlike a downed B-52, the location of a device would be unknown. And unlike past cases of nuclear extortion, there would be no warning.

“They’re interested in causing damage,” said Jeffrey Richelson, a senior fellow with the National Security Archive, a private research institute at George Washington University. “You aren’t going to get a note from al-Qaida saying that ‘we have a nuclear bomb or a dirty bomb in Los Angeles or in Chicago or Boston.’ “

DOE officials began “no notice” training exercises to test the government’s ability to mobilize rapidly. But such exercises could be effective only if intelligence was good enough to provide some warning of terrorists’ intentions. And well before the Sept. 11, 2001, terrorist attacks, experts warned that was not the case.

After the terrorist strikes, the DOE dispatched teams to conduct radiological surveys in as many as three major cities at the same time, for up to three weeks each, according to the DOE’s April 2002 supplemental budget request.

Linton Brooks, head of the NNSA, told Congress that July that by that summer, officials had “performed approximately 70 assessments” of possible nuclear-related incidents.

Around then, NNSA officials began breaking down the NEST bureaucracy into smaller teams, to shorten response times and decentralize expertise, officials said.

Although NNSA’s emergency response teams are part of the DOE, they can be summoned by officials at the Department of Homeland Security, the FBI and the State Department, officials said. They always operate in secrecy.

“You don’t want the bad guy to know that you are out there looking,” said Darwin Morgan, a spokesman at the Energy Department’s Nevada Operations Office. “Because if they have the means to cause it to detonate, we don’t want them to set it off.”