"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."