Mexican authorities say they found a shipment of highly radioactive waste near an abandoned stolen truck that was carrying the material.
Police say the thieves took the dangerous cobalt-60 out of its container and left it in a field near a rural farming town north of Mexico City. Experts say anyone exposed to the material could die within days.
Officials have sealed off the field and say there is no danger to people living nearby.
Thieves stole the truck from a gas station Tuesday in Mexico's Hidalgo state.
Officials say it is likely the hijackers only wanted the truck and had no idea of its deadly cargo.
The truck was carrying the cobalt-60 from a hospital in Tijuana to a waste facility near Mexico City.
Cobalt-60 is used largely for medical treatment. But experts say it could also be used in a so-called dirty bomb to spread radioactivity over a large area.