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."