A truck carrying a radioactive source is stolen in Mexico The material had been used for cancer treatments in Tijuana Mexico has told the International Atomic Energy Agency about the theft (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 that also can be used to make a so-called dirty bomb. The International Atomic Energy Agency announced the theft Wednesday. An early theory is that the thieves were unaware of what exactly they now possess. 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. 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 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 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 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. 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. 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.