Colombia said it seized at least 66 pounds (30 kg) of uranium from the country's biggest left-wing rebel group on Wednesday, the first time radioactive material has been linked to the four-decade-old guerrilla war.
The uranium was found in a rural area long considered a Marxist guerrilla stronghold just south of the capital Bogota.
It is being examined by government experts, the defense ministry said in a statement, although it did not say where the material came from or what it could be used for.
An expert on Colombia's cocaine-fueled conflict said rebels of the Revolutionary Armed Forces of Colombia, or FARC, do not have the facilities needed to make a bomb with uranium.
"This appears to have been part of a black market operation that the guerrillas were trying to use to make money," said Pablo Casas, an analyst at Bogota think-tank Security and Democracy.
"This is new for Colombia and could bring the FARC into the major leagues of black market terrorist transactions," he said.