WASHINGTON - Without realizing it, millions of American commuters are in close proximity every day to extremely hazardous nuclear material that terrorists would love to turn into weapons of mass destruction -- if they could.
It happens when we drive down roadways and pass certain cargo trucks. Or when we are near buildings under construction. Or receive cancer therapy treatments and x-rays. Or walk under household smoke detectors.
Radioactive material of various types is part of all kinds of everyday tools commonly used to kill cancer cells, build things and keep heart patients alive -- from cancer therapy and x-ray machines to laser drills and even obsolete heart pacemakers -- which are commonly transported between medical centers, construction sites and waste disposal areas.
In theory, a committed terrorist could break open the shielded containers of nuclear material inside larger medical devices and use it to make an improvised explosive device laden with radioactive ingredients, known as a "dirty bomb."
But they might die trying -- from radiation sickness -- and that may be why a dirty bomb has never been used.
"That's probably one of the major roadblocks to creating a dirty bomb, that the terrorist would be exposed to radiation," Dr. Ellen Carlin, a former WMD expert at the House Committee on Homeland Security, told ABC News on Wednesday.
Very few terrorists caught inside the U.S. since 9/11 succeeded in building a viable conventional IED, and only the Boston Marathon bombers actually detonated explosive devices.
Another top expert said that while unprecedented, it is possible.
"To me, it's a bit of a puzzle why it hasn't happened," Matthew Bunn, a nuclear expert at Harvard University's John F. Kennedy School of Government, said in an interview.
On Wednesday, the International Atomic Energy Agency reported that a VW truck in Mexico transporting an old cancer therapy machine containing highly hazardous Cobalt-60 was hijacked by unknown gunmen. A U.S. official briefed on the Mexico theft told ABC News that it appears that the truck, not its contents, was targeted by the thieves.
According to a 2007 U.S. Bureau of Transportation Statistics study, 515,000 tons of radioactive materials are transported in the U.S. per year. Most of that is by truck and the rest is by air and parcel.
Ever since federal officials revealed a decade ago that since-convicted al Qaeda operative Jose Padilla had been sent by 9/11 mastermind Khalid Shaikh Mohammed to the U.S. to attempt to make and detonate a dirty bomb, counter-terrorism officials and government scientists have been on watch for unusual signs of radioactivity in cities, and they've also studied whether these are viable weapons. In Padilla's case, the KSM "plot" was not viable and he made no known effort toward carrying it out.
In one demonstration for U.S. government officials of a radioactivity sniffer system in New York City last year, a commuter's vehicle was almost immediately flagged and pulled over. Inside the trunk was a laser drill – a harmless construction tool containing a small amount of radioactive material, according to a source who was there.