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.
As common as machines are that contain material that could possibly make an improvised explosive device "dirty" with radioactive material that would scare Americans more than it would kill any beyond the bomb blast itself, their transport is regulated by security requirements of the Nuclear Regulatory Commission. Of three categories of nuclear material in the U.S., the most dangerous -- Category 1 -- has never been stolen inside the United States, NRC officials told ABC News. The Mexican cargo would be considered Category 1.
Even outside the U.S. homeland, where similar medical and construction devices are used in more lawless countries where extremist groups might steal one more easily and exploit its lethal innards, "nobody has ever attempted to use a dirty bomb," Bunn said.
The problem for terrorists is that aspiration hasn't been matched with operational capability.
"You don't need to hijack a truck in Mexico to make a dirty bomb in the U.S. There is so much equipment in the U.S. with Cobalt-60 or other radiation sources," Carlin said. "It's extremely widespread. It's in every hospital and even veterinary clinics."
However, simply prying open a small device on Wall Street, for example, would likely pose minimal danger to anyone outside the immediate vicinity of the device. It also wouldn't have immediate health impact.
Placing radioactive material within an IED would conceivably spread it over a larger area and contaminate it. "That's the appeal of a dirty bomb," Carlin said.
The challenges of a terrorist personally handling or transferring any kind of radioactive material from a stolen medical machine to an IED and exploding it at a target site without exposing themselves to radiation in the process may in part explain why a dirty bomb has never been used, the experts agreed.
The NRC reports that during fiscal year 2013 there were no recorded incidents of unrecovered lost, abandoned or stolen nuclear material in the United States.
An NRC spokesman also said the agency has never received a report of something as dangerous as Cobalt-60, the material missing in Mexico, being stolen in the United States. More specifically, "there has never been a Category 1 source reported stolen in the U.S."
IAEA Category 1 materials "are considered to be the most 'dangerous' because they can pose a very high risk to human health if not managed safely and securely. An exposure of only a few minutes to an unshielded Category 1 source may be fatal." Category 3 incidents, like those involving the old pacemakers, are much less worrisome.
The NRC recorded only 10 serious incidents of lost, abandoned or stolen nuclear material in the United States in fiscal year 2012. All but one was recovered. The one missing is in the chest of someone who died, one of the few remaining people in the U.S. with plutonium-powered pacemakers.
"The body was buried without recovering the pacemaker," the NRC reports. "There is no planned action to recover the pacemaker from the buried patient."
With ABC News Producer Brian Hartman
ABC News' Brian Hartman contributed to this report.