"The thing I worry about most is nuclear weapons being in the hands of a group that does not have a return address. North Korea has a return address. The question is who they may sell the material to, or weapons to," Nunn says.
That's one danger. Another is a rogue state like North Korea funneling nuclear ingredients directly to them, CBS News chief investigative correspondent Armen Keteyian says.
"That's the raw material for terrorism, and it's not properly secured in many cases. Small amounts, but enough to make a crude weapon," Nunn says.
What alarms Nunn and others is the amount of weapons-grade uranium and plutonium available on the worldwide black market. Some sellers are working inside countries like Pakistan, others are working out of the trunk of a car.
In fact, the number of "confirmed incidents" of illegal trafficking in nuclear materials has grown from about 25 worldwide in 1996 to nearly five times that number two years ago, according to the International Atomic Energy Agency.
That's why the U.S. government is taking new steps to track nuclear ingredients.
"We're looking at instruments that can help us detect the movement of nuclear materials," says Dr. Ralph James of Brookhaven National Laboratory on Long Island, not far from New York City.
At Brookhaven National, James is developing a new generation of nuclear sensors meant to be deployed around the country at airports, seaports, bridges and tunnels.
It will be a difficult task. It's not easy to determine, often on the fly, the difference between gamma rays emitted from a pack of kitty litter and a dirty bomb being driven into the heart of a major U.S. city.
Nevertheless, James says his technology is not just scientific smoke and mirrors. "These detector technologies are plenty capable of detecting radiation," he says. "But I don't want to diminish the fact we're still trying to increase their sensitivity."
It's an increasingly urgent mission now that North Korea may be a nuclear country, one with a history of selling its weapons to any willing buyer.