In 2009, Western intelligence agencies discovered a clandestine underground facility called Fordow, near the city of Qom, that is said to be capable of housing 3,000 centrifuges for enriching uranium.
Israel worries that such facilities may be invulnerable to conventional bombing if Iran begins building a weapon. Israeli officials have warned that Iran could create what they call a "zone of immunity" by year's end.
And some U.S. officials have come to different conclusions about the intelligence. Among them is Rep. Mike Rogers, a Michigan Republican who is chairman of the House Intelligence Committee. "We know that [Iran is] aggressively pursuing a nuclear weapons program," Rogers said this month.
U.S. intelligence on Iran's nuclear ambitions has vacillated over the years. After Iranian dissidents exposed a long-hidden program in 2002, U.S. intelligence warned that Tehran was "determined to build nuclear weapons."
In 2006, Bush asked aides to present him with options for a U.S. attack. But newly recruited informants, intercepted conversations and notes from deliberations of Iranian officials led U.S. intelligence to reconsider its warning.
In December 2007, the National Intelligence Estimate judged with "high confidence" that Tehran had halted its nuclear weapons program in the fall of 2003. It judged with "moderate-to-high confidence that Tehran at a minimum is keeping open the option to develop nuclear weapons."
In his 2010 memoir, "Decision Points," Bush questioned whether analysts had reversed course to atone for their errors on Iraq.
Michael Hayden, who was CIA director in 2007, said the analysts who wrote the report had no political motivation. "It was intelligence professionals calling balls and strikes the way they saw them," he said in an interview.
He said the 2007 estimate was poorly worded and "quickly got translated into 'Iran stopped its nuclear program,'" which he does not believe is accurate.
The more important finding, Hadley said, was that Iran was continuing its efforts to develop fissile material and to build ballistic missiles capable of delivering warheads.
"They are doing everything they can to put themselves in a position so that they have a clear and fairly quick route to a nuclear weapon," he said.