Six men were being treated Friday for radiation symptoms at a central Mexico hospital where police are holding them in connection with the theft of radioactive material, officials said.

The suspects, who face charges of breaking environmental laws, are believed to have come into contact with a cancer-treating teletherapy device that contained cobalt-60, a dangerous radioactive isotope.

The medical machine, which had been used in a hospital in northwestern Mexico, was being transported by truck to a radioactive waste disposal facility when the vehicle was stolen at a service station on Monday in central Hidalgo state.

The capsule holding the cobalt-60 was abandoned in a rural area 70 kilometers (43 miles) north of Mexico City on Wednesday, about one kilometer (0.6 miles) from the truck.

Authorities have set up a security perimeter around the hospital where the suspects are being held.

Federal agents first brought a 16-year-old boy and a 25-year-old man to the hospital late Thursday with symptoms including vomiting, nausea and dizziness, said Hidalgo's under secretary of health Jose Antonio Copca Garcia.

"The two have serious health problems," he said.

The four others were brought into the hospital hours later.

A spokesman for the federal attorney general's office said the six face charges of transporting hazardous material.

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."