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.