(CNN) — A pair of thieves in Mexico may have stolen more than they bargained for when they targeted a truck this week.
The stolen vehicle was carrying a delicate cargo — a radioactive element used for medical purposes, but one that can be used to make a so-called dirty bomb.
The International Atomic Energy Agency announced the theft Wednesday.
Mexican authorities told the IAEA that the truck, which was transporting the cobalt-60 teletherapy source from a hospital in Tijuana to a radioactive waste storage center, was stolen Monday in Tepojaco, near Mexico City.
An early theory is that the thieves were unaware of what exactly they now possess.
“At the time the truck was stolen, the source was properly shielded,” the IAEA said. “However, the source could be extremely dangerous to a person if removed from the shielding, or if it was damaged.”
The U.S. Department of Homeland Security is assisting with the investigation into the stolen truck, Mexican authorities said.
Experts consider cobalt-60 one of the “candidates” for making dirty bombs.
Bombs made with cobalt-60 “pose a threat mainly because even a fraction of a gram emits a huge number of high-energy gamma rays; such material is harmful whether outside or inside the body,” according to a 2011 report by the Congressional Research Center.
In a speech last year, the IAEA director warned that such a dirty bomb “detonated in a major city could cause mass panic, as well as serious economic and environmental consequences.”
The preliminary information suggests that the thieves did not know the truck’s cargo when they stole it, said Jaime Aguirre Gomez, deputy director of radiological security at the National Commission for Nuclear Security and Safeguards.
The radioactive material had been used in radiotherapy for cancer treatments at a Tijuana hospital and is now in disuse, he said.
The shielding that protects the cobalt-60 is designed so that the radioactive source is difficult to extract, Aguirre said. The casing is designed not to be opened or perforated easily.
The driver of the white 2007 Volkswagen truck had stopped for rest at a gas station, the state-run Notimex news agency reported, and early in the morning, two men with a gun accosted him.
The men forced the driver out of the truck and tied him up, Notimex reported.
Mexico alerted the IAEA to the theft, following international protocol, Aguirre said.
Cobalt-60 is used in radiotherapy and in industrial tools such as leveling devices and thickness gauges. Large sources of cobalt-60 are used to sterilize certain foods, as the gamma rays kill bacteria but don’t damage the product, according to the Environmental Protection Agency.
If released into the environment, the radioactive material can harm people.
According to the Congressional Research Center report, in Thailand in 2000, a disused cobalt-60 source was stored outdoors and bought by two scrap collectors, who took it to a junkyard where it was cut open.
Some workers suffered burn-like injuries, and eventually three people died and seven others suffered radiation injuries, the report says. Nearly 2,000 others who lived nearby were exposed to radiation.
Cobalt-60 has a half-life of 5.27 years.
CNN’s Carloo Perez, Evan Perez, Holly Yan and Christine Theodorou contributed to this report.