Bogota said information about the uranium was found in computer files obtained following a cross-border raid into Ecuador, which killed Raul Reyes, a senior Farc commander, and several other fighters.
The March 1 raid sparked a major diplomatic crisis in the region, with Ecuador and Venezuela temporarily breaking diplomatic ties with the Colombian government, led by Alvaro Uribe, and sending troops to their Colombian borders.
Freddy Padilla, Colombia's most senior military commander, said on Wednesday that two informants who had access to a contact known as "Belisario" alerted military intelligence to the Farc's possession of radioactive material.
"We expect in the next few days that they will ... tell us the source and other details about the material that the Farc had been trying to acquire"
Freddy Padilla, Colombian military commander
"Belisario" is the person mentioned in Reyes' computer as having access to uranium.
"We feel that the fact that we confiscated this material is of great importance, given that we have once again kept the Farc from being able to use this radioactive material which they have been trying to obtain since October of 2005," he said.
"We expect in the next few days that they will, based on technical analysis, tell us the source and other details about the material that the Farc had been trying to acquire."
Three weeks ago General Oscar Naranjo, Colombia's police chief, said computers seized at Ecuador camp showed that Farc had obtained about 50kg of uranium.
However the Farc had issued a statement rejected the claim it had anything to do with the uranium, as they said they lacked the means to process the material.
"Only developed countries, like the United States and others, have the required conditions and technology to process uranium, and not guerrillas who are fighting for the dignity of a people with rifles and even sticks," it said in a statement quoted by EFE news agency.
Experts at Ingeominas, the Colombian state geological institute, have since confirmed that the seized is depleted uranium (DU), the defence ministry said.
DU is a residue of the enriching and reprocessing of uranium. It has a low level of radioactivity and can reportedly be used to make missiles.
It is not clear what the group planned to do with their alleged uranium purchase, but some diplomats suggested it was to be used for trading purposes.