Nuclear attribution is the process of tracing the origin of nuclear material that has been used in a nuclear explosion. The problem is not necessarily a straightforward one, for it may be possible to obtain nuclear precursors through the black market, and therefore relatively anonymously. Attribution to the source is desirable due to the trend of nuclear proliferation. Nuclear capability has expanded from only a couple of states, to where a terrorist group could make a bomb with only partial state backing. The United States has demonstrated its interest in nuclear attribution by passing the Nuclear Forensics and Attribution Act, which calls for the development of attribution capabilities.