A United Nations report, outlining the interception last October, said there is "no doubt" the smuggled uranium-238 came from mines in the Democratic Republic of the Congo's mineral-rich Katanga province.
The smuggled uranium discovered by Tanzanian customs agents was hidden in shipment of coltan, a rare mineral used to make chips in mobile telephones. According to the manifest, the coltan was to be smelted in the former Soviet republic of Kazakhstan after being shipped to Bandar Abbas, Iran's largest port.
"There were several containers due to be shipped and they were all routinely scanned with a Geiger counter," one customs official said.
"This one was very radioactive. When we opened the container it was full of drums of coltan. Each drum contains about 50 kilograms of ore. When the first and second rows were removed, the ones after that were found to be drums of uranium."
Uranium-238, when used in a nuclear reactor, can be used to create plutonium for nuclear weapons.
"The container was put in a secure part of the port and it was later taken away, by the Americans, I think, or at least with their help," he said. "We have all been told not to talk to anyone about this."
According to the U.N. report, which has been submitted to the sanctions committee, Tanzania provided "limited data" on three other shipments of radioactive materials seized over the past ten years.
"In reference to the last shipment from October 2005," the report reads, "the Tanzanian government left no doubt that the uranium was transported from [Katanga province] by road through Zambia to the United Republic of Tanzania."