Mexican soldiers Wednesday set up a safety perimeter around a cancer-treating device containing dangerous radioactive material that was stolen along with a truck from a gas station.
The people who stole the truck and removed the device from a steel-reinforced wooden box and left it in a rural area north of Mexico City are probably already dead or dying, the national nuclear safety board said.
But the danger of contamination is minimal because the area where the device was found is so uninhabited, it added. No evacuations were necessary.
There was no immediate word on who might have stolen the truck. It was on its way to dispose of the disused medical device at a nuclear storage facility.
Experts are trying to figure out the best way to recover the device safely, the National Commission for Nuclear Safety and Safeguards (CNSNS) said in a statement Wednesday evening.
The thieves apparently just wanted the truck, which was stolen Monday, without knowing about the cargo it carried, officials said.
The device containing cobalt-60 was taken out of its container and left hundreds of meters (yards) from the truck in Hueypoxtla, said Mardonio Jimenez, operations director at the CNSNS.
"It's almost absolutely certain that whoever removed this material by hand is either already dead or about to die," CNSNS director Juan Eibenschutz told Milenio television.
Eibenschutz said the transport company failed to live up to its commitment, saying the truck lacked a tracking device or proper security despite the firm's experience. He said the matter should be investigated.
The white Volkswagen Worker truck was transporting the device from a hospital in the northwestern city of Tijuana when it was stolen at a service station in central Hidalgo state.
The vehicle was supposed to deliver the material to a radioactive waste disposal facility in the central state named Mexico.
The International Atomic Energy Agency warned that the material was "extremely dangerous" if removed from its shielding. Experts also said the 60 grams of cobalt-60 inside it was enough to make a "dirty bomb", designed to spread radioactivity.
Authorities had searched for the truck in six states and the capital, delivering radio messages for people to call an emergency number in case they saw the truck.
The driver told investigators that two gunmen approached him at a Pemex service station, tied him up and drove away with the truck, according to a text of the testimony shown by the Hidalgo state prosecutor's office.
The manager of the Pemex service station, an hour's drive north of Mexico City, told AFP the driver appeared to have parked across the street to rest overnight.
The material was on its way to the Radioactive Waste Storage Center in Maquixco, Mexico state. The facility is surrounded by a white fence topped with barbed wire, but no armed guards were visible outside, an AFP correspondent said.
An official from the center said the truck driver had been waiting for the facility to open at 8:00 am on Tuesday.
Mexico's drug cartels have diversified their illegal activities in recent years, stealing oil and minerals, but officials have not said who the cobalt-60 thieves might be.
'Sufficient' for dirty bomb
Experts have long warned about the risks posed by the large amounts of radioactive material held in hospitals, university campuses and factories, often with little or no security measures to prevent them being stolen.
In an incident involving a teletherapy device in Thailand in 2000, 425 Curies -- the measure of radioactivity -- of cobalt-60 was sufficient to make 10 people very ill, three of whom died, according to the IAEA.
The equipment stolen in Mexico contained nearly 3,000 Curies, CNSNS radiological security director Jaime Aguirre Gomez told AFP.
Cobalt-60 is a radioactive isotope of the metallic element cobalt and the gamma rays it emits destroy tumors. But contact or just being near it can cause cancer if not properly handled and sealed.
More worryingly, though, such material could in theory be put in a so-called "dirty bomb" -- an explosive device designed to spread the radioactive material over a wide area.
The quantity stolen in Mexico was "sufficient" to make a dirty bomb, said Michelle Cann, an analyst at the Partnership for Global Security.
"But the ultimate level of damage and contamination hinges on many factors," she said.