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.