If you live in a big city, chances are a secret government team has been in your neighborhood hunting for radioactive material -- even terrorist bombs. But you probably didn't even notice.
ABC News' Cynthia McFadden got an exclusive, inside look at one of the most secretive units working in the war on terror -- a little-known government SWAT team called the Nuclear Emergency Support Team.
"We look like normal people out there. Miniskirts and flip-flops and baseball hats," said one female NEST member, who asked not to be identified for security reasons.
The unit serves as a sort of "Ghostbusters" for nuclear bombs, often scouring major events such as Super Bowls or Olympic competitions for signs of trouble.
They hide their detection equipment in briefcases, knapsacks, even beer coolers, and travel in mobile labs disguised as ordinary delivery vans. They often work right out in the open, but remain hidden from the untrained eye.
The woman next to you in the ballpark, the executive at the airport, the man with the golf bag -- any of them could be carrying sophisticated, well-disguised radiation detectors.
NEST is made up of nuclear physicists and scientists who work in the nation's weapons labs, but when their pagers alert them, they become an investigative unit tasked with finding a terrorist's nuclear weapon before it explodes.
Though they're often chasing dangerous characters, NEST members carry technical equipment rather than weapons.
"That is why we are attached at the hip to law enforcement," said Debbie Wilbur, who heads NEST for the Department of Energy's Nuclear Security Administration. "They understand the risks. These guys run toward the problem. Everybody else is hightailing it out of there."
To see what they do, ABC News went to Las Vegas last summer to get a rare glimpse of a NEST team in action at its headquarters at Nellis Air Force Base.
As a drill, a team of NEST investigators was asked to search the grounds of the base for a small amount of cobalt-60 -- a highly radioactive material that can be deadly if used in a dirty bomb. They piled into a NEST van packed with high-tech equipment to begin the search.
The cobalt-60 had been hidden in a nearby parking lot, and the highly-sensitive detection equipment in the van began beeping soon after the search began. Background radiation from construction equipment, granite or even just the Earth can register alerts for an elevated radiation level. The challenge for the team is to determine which hit is the real threat.
The parking lot was filled with more than 100 cars, and the team drove the van past each one. Every move the team made was transmitted and recorded back at the base. After 15 minutes, several beeps sounded, indicating a real hit.
The hit was radioed back to the base, and the team returned to explore the area on foot. Once the correct vehicle was identified, a team member used another piece of equipment, known as an "identifier," to determine what type of material was in the car.
"And it identifies this — cobalt-60," a male team member said. "Yes, there's definitely a radiation source."
NEST also has technology that allows the teams to detect radiation from the air. A test run at the Nevada site — considered ground zero for America's atomic bomb tests — showed how a helicopter flying at a low altitude was able to find a small amount of cesium, a rare element, in a stretch of desert. The detection methods and technology are state of the art, but some experts say it may not be good enough.
Peter Zimmerman, a nuclear physicist who was formerly the chief scientist for the Senate Foreign Relations Committee, pointed out that even with NEST's high-tech tools, it is very difficult to find nuclear material.
"The bomb is likely to be shielded by the walls of the building, could be in a basement, could be shielded by real shielding," Zimmerman said.
And background radiation that surrounds many things can create significant problems, often making it nearly impossible to detect the nuclear material in a bomb.
"It's there. It masks the signal, it mimics the signal. It makes it more [difficult], I think even, than a needle in a stack of needles. I once heard it described as the drops from a glass of vodka in a thunderstorm," Zimmerman said.
He noted that the helicopter experiment in the Nevada desert might not be a true example of how difficult it is to find nuclear material in a crowded city, like Los Angeles. And he said a material like cesium might send off a stronger signal that some other bomb-making materials.
"Cesium's a strong gamma emitter. It's hard, it's very hard to shield it," he said.
In fact, sources like cobalt or cesium, which could be used in a dirty bomb, emit strong radioactive signals and are relatively easy to detect. But plutonium and highly enriched uranium, the two fuels used in nuclear weapons, are far less radioactive even though they're potentially more dangerous.
Zimmerman said uranium 235, which is used to make bombs, is a very low-emission material, making it hard to find. Its emission levels are so low that something as simple as a piece of aluminum foil could mask it.
Because of this, the technology alone is unlikely to just chance upon loose nuclear material. The investigators must first know where to look. Without good intelligence information, finding threatening nuclear material is a very difficult task.
"I won't say virtually impossible. Without good intelligence, it's extremely difficult," Zimmerman said.
In 1974, Boston police received a ransom letter that said an atomic bomb had been planted somewhere in the city. Experts were flown in to search for the device, but the response was poorly organized, and their equipment ended up at the wrong airport.
The organizational failures surrounding the Boston incident led to the creation of NEST. Over the next decade, NEST responded to dozens of nuclear extortion threats. But responding to a ransom threat, which gives investigators time to search while the extortionists wait for a payoff, is far different than the threat posed by terrorists.
The attacks of Sept. 11, 2001, offered a good example of the challenges faced when responding to immediate danger. After the World Trade Center collapsed, NEST investigators were stranded in Las Vegas, unable to respond for 24 hours because their specially equipped plane was grounded along with almost every other aircraft in the country.
"We realized after the towers were hit, the Pentagon was hit, that we were dealing with a situation in which we didn't have the time, like an extortion event," Wilbur said.
After 9/11, NEST created smaller teams that could respond faster, and they worked around the clock, going from one city to another, searching up to three cities at a time.
"There are just a lot of threats out there that we never even considered before," one male team member said.
Today, with close to 1,000 team members in 29 locations, two helicopters and three planes, NEST teams deploy dozens of times each year on search drills in cities designated by the FBI. They launch into action when the Homeland Security Department raises the threat level.
"We keep a bag packed. Often we don't know exactly where we're going. They put us on a plane and where we land is ... is where we land," the female team member said.
ABC News for the first time observed an actual drill at Andrews Air Force base in Maryland. From the moment they were alerted, the NEST team had four hours to deploy. They loaded up water, tents, protective vests and gear that protects from chemical and biological agents, as well as detection gear and communications equipment -- anything they would need to survive in the field.
The Maryland team was ready and out the door in under an hour, well ahead of schedule. It's a tough job, with very dangerous work and hectic schedules. NEST investigators are often away from home for weeks at a time, though they earn just a few extra dollars a day when they're on call.
So why do they do it?
"I'll tell you this. When the pager goes off and somebody says, 'The U.S. has a problem, and you're the one we've selected to go,' there's nothing that beats that feeling. That makes everything else worth it," one member of the Maryland team said.
Government officials concede that the NEST operations are not foolproof, but the combination of technology, detailed intelligence and dedicated investigators working with law enforcement forms a complex network fighting to stop a terrorist bomb from killing Americans.
The simple fact remains that there are literally tons of nuclear materials in more than 40 countries around the world. It only takes a few kilograms for terrorists to make a bomb and threaten lives. To ensure 100 percent safety, the government and NEST investigators would have to make sure that all of this potentially lethal material doesn't fall into the wrong hands.
"You can't fight the laws of physics, but you can push as far as they'll let you go," said Ambassador Linton Brooks, head of the National Nuclear Security Administration.
"We would certainly be less safe if our opponents knew that we had no capability and that there was no barrier, no equipment, no team to stop them. After all, if the bad guy knows we have NEST, he may even decide that he can't do whatever bad thing, nuclear terror, because he can't beat NEST."