A robot brought in to pick up the material and deposit it in a safe container hasn't been able to reach the cobalt because bales of corn stalks have blocked the way and workers are still clearing a path, said Juan Eibenschutz, director general of the National Commission of Nuclear Safety and Safeguards.
"The terrain it's uneven and we have run tests to make sure we can reach the material," Eibenschutz said when explaining why it is taking so long to recover the material.
A farmer is being checked at a hospital after showing signs of radiation exposure, Eibenschutz said.
The man, who lives in the nearby farming town of Hueypoxtla, told authorities that he handled the material after finding it in the field and started feeling sick soon after, Eibenschutz said.
The cobalt-60 was being transported by a truck that was stolen early Dec. 2 from a gas station in Hidalgo state. The thieves dumped the material in the field about a kilometer (a half a mile) from Hueypoxtla, a town of about 4,000 people in neighboring Mexico state. Officials have said the cobalt poses no threat to the town, and is dangerous only in close proximity.
The truck was taking the cobalt to a nuclear waste facility in the state of Mexico, which is adjacent to Mexico City. The material had been removed from obsolete medical equipment used in radiation therapy.
Eibenshutz's comments came a day after a federal judge ordered five people held for 40 days under a form of house arrest pending possible charges.
Four of the detainees are suspected in the theft of the truck and the fifth was allegedly a possible buyer of the stolen vehicle, said an official with the Attorney General's Office, who spoke on condition of anonymity because he was not authorized to speak to the media.
The five detainees and a 16-year-old who was questioned and released were tested for possible radiation exposure and turned over to federal authorities after being released from a hospital.
The farmer exposed to radiation is not a suspect in the case, authorities said.