"Based on the information available, the Mexican authorities and the IAEA believe the general public is safe and will remain safe," the International Atomic Energy Agency said in a statement.
Mexican authorities said yesterday that they had found the cancer-treating device containing cobalt-60 in a rural area north of Mexico City, two days after the truck taking it from a hospital to a waste storage centre was stolen at a petrol station.
CNSNS director Juan Eibenschutz told Milenio television that it was "absolutely certain that whoever removed this material by hand is either already dead or about to die."
The IAEA said the material was "extremely dangerous" and if not securely protected "would be likely to cause permanent injury to a person who handled it or who was otherwise in contact with it for more than a few minutes."
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The Vienna-based body added however that people exposed to the radioactive substance "do not represent a contamination risk to others."
The incident was a reminder of the dangers posed by the huge amounts of nuclear materials in hospitals and industry around the world if they are not handled properly and with sufficient security.
In particular there are fears that extremists could steal the material and put it in a so-called "dirty bomb" -- an explosive device spreading radioactivity over a wide area and sparking mass panic.