A remotely-operated arm on the robot removed the cobalt-60 from a capsule that had been opened during the theft and placed it into a concrete-shielded container on another vehicle, El Universal reported Wednesday.
The truck and the radioactive material, which is used in medical procedures, were found Dec. 4 near Hueypoxtla, about 25 miles from where the truck was reported stolen.
Some 100 Federal Police maintained a two-mile security perimeter around the area until the cobalt-60 could be safely stored in the shielded container.
A man who reportedly had contact with the stolen capsule, Francisco Sanchez Angeles, 41, was admitted to a hospital Tuesday for examination.
His wife said Sanchez had carried the capsule on his shoulder to a wooded hill, where he hid the container in straw.
Hospital officials said Sanchez was vomiting and had burns on a shoulder when he was admitted.