In New York, cops armed with Geiger counters pull over trucks for random inspections.
Robotic underwater cameras crawl along the hulls of cruise ships looking for explosives and traces of radioactivity.
And from the air, sensors snoop for radiation hot spots. It is the last line of defense against an unthinkable threat.
"Terrorists — al Qaeda, bin Laden — have sought nuclear materials for a number of years now," FBI Director Robert Mueller told CBS News correspondent Bob Orr in an exclusive interview.
Mueller says terrorists would like nothing better than to hit the U.S. with a nuclear weapon.
"When we saw 9/11, everyone I think pretty much looking at those pictures thought this is about as bad as it could get," Mueller said. "Its not as bad as it could get. A nuclear device — if a nuclear device went off, you're talking about devastation that is far, far beyond what we saw on September 11th."
That kind of devastation was portrayed in the film "The Sum of All Fears," when a nuclear bomb leveled much of Baltimore.
That was just a movie, but officials warn the threat is real, and the bomb wouldn't have to be that big. A small nuclear device packed inside a case and left in the heart of Washington could take out the White House and everything else within a square mile.
"It could kill tens of thousands of people, and that's why it is a tremendously serious matter for us to address," Mueller said.
Officials say it would not be easy for terrorists to get a nuke or enough material and know-how to make one of their own. But thousands of Cold War weapons remain in Russian stockpiles, and emerging nuclear powers such as Pakistan, Iran, and North Korea could make nuclear fuel more readily available for terrorists.
Since the early '90s, the United Nations' atomic watchdog has confirmed at least 18 cases of illegal trafficking of weapons-grade uranium on the black market. Most recently a Russian man was arrested for trying to sell the bomb material to a radical Islamic terrorist.
Intercepting the material, and keeping it from the terrorists, is the first goal. Nobody at home wants to deal with an explosion after the fact.
"This is a big deal," said New York Police Commissioner Ray Kelly. "God forbid something like this happens; we have to be as prepared as we possibly can be and do everything we can to deter and detect."
To date, al Qaeda has not been able obtain nukes, but Mueller says they're still desperate to get one.
"It's not hype. It's something we deal with day in and day out," Mueller said. "When you are talking about an improvised nuclear device, it is something that would be horrifying if it fell into the hands of terrorists or terrorists were able to manufacture such a device — and we can not let that happen. We just can not let that happen."
An unthinkable threat — not an impossible one.