Of the detained men, ages 16 to 38, only the 16-year-old showed signs of radiation exposure and he was in good health, a spokeswoman for Hidalgo's Health Department yesterday said on condition of anonymity because she isn't allowed to discuss the case.
The six were detained on Thursday as part of the investigation and taken to the general hospital in Pachuca for testing, an official with federal prosecutors said.
Officials have not said what roles the six allegedly had in the theft.
Hidalgo state Health Minister Pedro Luis Noble said the men suffered from skin irritations and dizziness, but that none were in serious condition. Only one was vomiting, a sign of radiation poisoning.
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The theft triggered alerts in six Mexican states and Mexico City, as well as international notifications to the US and the International Atomic Energy Agency in Vienna. It raised concerns that the material could have been stolen to make a dirty bomb, a conventional explosive that disseminates radioactive material.
But Mexican officials said that the thieves seemed to have targeted the cargo truck, which has a movable platform and crane, and likely didn't know about the dangerous cargo.
The six were arrested by Hidalgo state police, said state attorney general's spokesman Fernando Hidalgo.
The driver of the truck, who had stopped to rest at a gas station early on Monday when the theft occurred, said two armed men made him get out, tied his hands and feet and left him in a vacant lot. Hidalgo said he didn't know how or if the others were involved.
The cobalt-60 was from a hospital in the northern city of Tijuana and was being transported to nuclear waste facility in the state of Mexico, which borders Mexico City.