"[…] Iran's construction of gas centrifuge uranium enrichment facilities is currently the main source of proliferation concern. Gas centrifuges can produce both low-enriched uranium (LEU), which can be used in nuclear power reactors, and weapons-grade highly enriched uranium (HEU), which is one of the two types of fissile material used in nuclear weapons. The United States has assessed that Tehran has technological and industrial capacity to produce nuclear weapons, but has not yet mastered all of the necessary technologies for building such weapons. Whether Iran has a viable design for a nuclear weapon is unclear. A National Intelligence Estimate made public in 2007 assessed that Tehran 'halted its nuclear weapons program' in 2003. The estimate, however, also assessed that Tehran is 'keeping open the option to develop nuclear weapons' and that any decision to end a nuclear weapons program is 'inherently reversible.' U.S. intelligence officials have reaffirmed this judgment on several occasions. Director of National Intelligence James Clapper testified before Congress in February 2016 that '[w]e do not know whether Iran will eventually decide to build nuclear weapons.' […] Iran and the IAEA [International Atomic Energy Agency] agreed in 2007 on a work plan to clarify outstanding questions regarding Tehran's nuclear program, most of which concerned possible Iranian procurement activities and research directly applicable to nuclear weapons development. A December 2015, report to the IAEA Board of Governors from agency Director-General Yukiya Amano contains the IAEA's 'final assessment on the resolution' of these outstanding issues."
CRS Report for Congress, RL34544
Federation of American Scientists: http://www.fas.org/sgp/crs/index.html