The truck was carrying old medical equipment used for radiation therapy. Inside was a sealed lead container, containing a capsule of radioactive cobalt-60.
The truck was on its way from a Tijuana hospital to a waste storage center near Mexico City when it was stolen from a gas station.
Late Wednesday, officials found the truck, but the container was opened and empty.
Crews found the radioactive material about half a mile away.
"This isn't like radioactive material stolen from a nuclear power plant," said ABC News consultant and former FBI agent Brad Garrett. "It doesn't sound like with what little information we have, that this is something you would steal if you were going to make a radioactive bomb."
So far, officials say there is no risk to the surrounding population from the radioactive material.