Mexican authorities have formally arrested five men suspected of stealing a truck carrying highly radioactive material in a theft that caused international alarm, an official said Monday.

A federal judge on Sunday ordered the five suspects held for 40 days on allegations of theft through organized crime and illegally transporting hazardous material, an official at the attorney general's office told AFP.

Four of the men are accused of stealing the truck while the fifth suspect bought it, the official said on condition of anonymity.

A 16-year-old boy who accompanied the buyer was placed under guardianship with juvenile justice authorities.

The six were hospitalized last week in the central state of Hidalgo with signs of radiation exposure, but they were given the all clear after 24 hours.

The truck was carrying an obsolete cancer-treating medical device that was on its way to a radioactive waste storage facility when it was stolen at a service station in Hidalgo on December 2.

The truck and device were found 65 kilometers (40 miles) north of Mexico City after a two-day manhunt.

The machine contained 60 grams of cobalt-60, a highly radioactive isotope that experts say could be used to make a crude "dirty bomb."

The theft prompted the International Atomic Energy Agency to issue an alert for "extremely dangerous" material and US authorities to keep tabs on the situation.

But Mexican authorities believe the thieves had merely planned to sell the device as scrap metal.