Mexican authorities on Wednesday recovered a truck and the radioactive medical equipment it was hauling to a waste facility when gunmen stole it from a gas station two days ago.
The radioactive material, cobalt-60, was found about a half-mile from the truck and its empty protective lead container near Mexico City, said Juan Eibenschutz, director general of the National Commission of Nuclear Safety and Safeguards.
Radioactivity was detected in the area near Hueypoxtla, an agricultural town of about 4,000 people. Authorities cordoned off the unpopulated area and said the material posed no danger.
The radiotherapy material used in cancer treatment "could be extremely dangerous to a person if removed from the shielding, or if it was damaged," the International Atomic Energy Agency said earlier.
Direct exposure to the radioactive isotope would result in death within a few minutes, Eibenschutz told the Associated Press.
"This is a radioactive source that is very strong," Eibenschutz said, adding that it can be almost immediately fatal, depending on proximity. "The intensity is very big if it is broken."
Authorities have not said whether any suspects have been found, dead or alive.
Eibenschutz said nothing indicated the thieves had targeted the material; they most likely wanted the white 2007 Volkswagen cargo vehicle with a movable platform and crane.
The material could not be used to make a nuclear bomb, but could be used in a dirty bomb, a conventional explosive that disseminates radioactive material, he said.
Eibenschutz didn't know the exact weight but said that it was the largest amount stolen in recent memory, and the intensity of the material caused the alert. Local, state and federal authorities, including the military, are searching for the truck.
The material was used for obsolete radiation therapy equipment that is being replaced throughout Mexico's public health system. It was coming from the general hospital in Tijuana when it was stolen.
The truck marked "Transportes Ortiz" left Tijuana on Nov. 28 and was headed to the storage facility when it stopped at a gas station in Tepojaco, in Hidalgo state north of Mexico City, driver Valentin Escamilla Ortiz told authorities.
He said he was sleeping in the truck when two men armed with a gun approached about 1:30 a.m. Tuesday. They made him get out, tied his hands and feet and left him in a vacant lot nearby.
When he was able to free himself, he ran back to the gas station to get help
The truck has a GPS locator, but it wasn't active at the time of the theft.
"Our suspicion is that they had no idea what they had stolen. This is an area where robberies are common," Fernando Hidalgo, spokesman for the Hidalgo state prosecutor, told Reuters.
Authorities sent out an alert to six central states and the capital, and Mexican customs officials were on alert to prevent the truck from crossing the border. All of the U.S. ports of entry have radiation detectors in place, and trucks crossing the border are routinely screened for radiation.
Such unwitting thefts of radioactive materials "are not uncommon," IAEA spokeswoman Gill Tudor told NBC News.
"In some cases, for example, radioactive sources have ended up being sold as scrap, causing serious health consequences for people who unknowingly come into contact with it," he said.
Three people died in Thailand in 2000 after a cobalt-60 teletherapy unit was sold as scrap metal and dumped in a junkyard. The IAEA said about 1,870 people living nearby were exposed to "some elevated level of radiation."
The United Nations nuclear watchdog says the agency annually receives more than 100 reports of thefts and other unauthorized activities involving of nuclear or radioactive materials.
Contributing: The Associated Press