MEXICO (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 delicate cargo -- a radioactive element used for medical purposes that also can be used to make a so-called dirty bomb.
Mexican authorities said they'd found the stolen truck and at least some of the radioactive cobalt Wednesday in a remote area about 40 km (25 miles) away from where it was taken.
But officials aren't sure whether any of the cobalt is missing, said Juan Eibenschutz Hartman, head of Mexico's National Commission for Nuclear Security and Safeguards.
And the suspected thieves are still on the loose, he said, though authorities expect they could turn up at a clinic suffering symptoms of radiation exposure.
The container holding cobalt was found about a kilometer away from the truck and had been opened, he said.
Authorities are guarding the area and have set up a 500-meter perimeter around it, Eibenschutz said. They are evaluating whether any residents were exposed.
Cleaning up the area could take weeks, he said, because they don't have robotic equipment they would need to quickly collect the dangerous cobalt. They're coming up with a plan and considering asking for help from the International Atomic Energy Agency, the United States or Canada.
The IAEA announced the theft on Wednesday.
Mexican authorities told the IAEA that the truck, which was transporting cobalt-60 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 had taken.
"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."
But Eibenschutz said the truck wasn't properly set up to transport the radioactive material, since it didn't have a GPS for tracking or other necessary equipment.
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 U.S. Environmental Protection Agency.
If released into the environment, the radioactive material can harm people. And 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 Service.
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."
Preliminary information suggests that the thieves did not know what the truck's cargo was when they stole it, said Jaime Aguirre Gomez, deputy director of radiological security at the National Commission for Nuclear Security and Safeguards.
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 truck and its cargo went missing early Monday after the driver of the white 2007 Volkswagen truck and an assistant had stopped to rest at a gas station, local prosecutor Marcos Morales told CNN.
At around 1 a.m. Monday, a man armed with a handgun knocked on the passenger window. When the passenger rolled down his window, the gunman demanded the keys to the vehicle, Morales said.
Both the driver and his assistant were taken to an empty lot where they were bound and told not to move. They heard one of the assailants use a walkie-talkie type device or phone to tell someone, "It's done," Morales said.
Mexico alerted the IAEA to the theft, following international protocol, Aguirre said. The U.S. Department of Homeland Security is assisting with the investigation into the stolen truck, Mexican authorities said.
The U.S. government has sensors at border crossings and sea ports to prevent radioactive materials from entering the country. This includes large stationary sensors designed to scan vehicles going through land border crossings as well as pager-size devices carried by agents.
Some of this equipment is sensitive enough that it has been set off by people who had recently undergone radiation therapy, according to a U.S. law-enforcement source.
According to the Congressional Research Service 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.