"The United States has been a leader of worldwide efforts to prevent the spread of nuclear weapons. To this end, the international community and many individual states have agreed to a range of treaties, laws, and agreements, known collectively as the nuclear nonproliferation regime, aimed at keeping nations that do not have nuclear weapons from acquiring them. The nonproliferation regime has also been concerned with preventing terrorists from obtaining a nuclear weapon or the materials to craft one. The attacks on New York and Washington September 11 added a new level of reality to the threat that terrorists might acquire a nuclear weapon and explode it in a populated area. […] The terrorist attacks of September 11 added the suddenly more realistic threat of an even more unimaginable assault with a nuclear explosive. While terrorists have not been ignored in nonproliferation efforts, particularly with regard to Russian nuclear materials, the major focus has been on preventing nation states from developing weapons capabilities. While many features of the nonproliferation regime, such as export controls and monitoring, are applicable to the terrorist threat, some shift in focus may be necessary. Numerous U.S. agencies have programs related to nuclear nonproliferation, but the major activities are carried out by the Departments of State, Defense, and Energy. DOE's [Department of Energy] program is part of the National Nuclear Security Administration, which is responsible for the management of the U.S. nuclear weapons program."
CRS Issue Brief for Congress, IB10091