Energy storage
According to episode "A.K.A.", the attacks destroyed twenty-three major American cities. The nuclear material used for creating the bombs in the attacks were brought in by the U.S. government. After the Soviet Union fell, neglected military and the satellite republics started selling nuclear material on the black market. So the Central Intelligence Agency started an operation: "Project Red Bell". The objective of the project was to obtain that material by any and all means necessary, but a few years before the events of the series a shipment disappeared on its way to the Department of Energy storage compound in Oak Ridge.