"Iran's nuclear program began during the 1950s. The United States has expressed concern since the mid-1970s that Tehran might develop nuclear weapons. 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 possesses the 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."
CRS Report for Congress, RL34544
Congressional Research Service: https://crsreports.congress.gov/