Federal police were blocking access Friday to a hospital in Pachuca, Mexico, where five adults and one teenager were said to be hospitalized with radiation symptoms.
An official familiar with the case confirmed to The Associated Press that the six may have been exposed to the cobalt-60 stolen from a truck in Hidalgo state, of which Pachuca is the capital, earlier this week.
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Hidalgo Public Security Director Alfredo Abed told El Universal newspaper that two of the six, 16- and 25-year-old males, were suspected of taking part in the armed hijacking at a highway gas station just after midnight on Monday.
After an intense search, police recovered the truck and the cobalt-60 on Wednesday in a farm town to the west of Pachuca and a few dozen miles outside Mexico City. Town officials told GlobalPost that several dozen policement and villages thought to be exposed to the cobalt-60 in the town had been tested and declared free of radiation poisoning.
The six were hospitalized Friday with symptoms including vomiting, nausea and dizziness, but authorities did not make it clear whether they were suspected in the theft.
Two of them were admitted Thursday and four more on Friday.
More from GlobalPost: Mexico: Cobalt responders are safe from radiation
The National Commission on Nuclear Safety and Safeguards has warned that whoever touched the radioactive source would likely die quickly.
The International Atomic Energy Agency has warned that the material, which was on its way to a radioactive waste storage facility, was "extremely dangerous."
The cobalt-60 had been removed from obsolete radiation therapy equipment at a government hospital in Tijuana and was being transported to a facility outside Mexico City for safe storage.
Senior Correspondent Dudley Althaus and AFP news wire contributed to this report.