Last spring, as U.S. intelligence agencies worked to complete an assessment of Iran's nuclear weapons program, they were firmly on track to reach the same conclusion as previous reports: Tehran was bent on building the bomb.

But within weeks, there was an abrupt change of course. The earlier drafts were scrapped. Analysts began to assemble a new report built around the single, startling conclusion that Iran's nuclear weapons program had actually been shut down for four years.

What happened?

As U.S. intelligence officials sought Tuesday to explain the remarkable reversal, they pointed to two factors: the emergence of crucial information over the summer, and a determination to avoid repeating the mistakes that preceded the Iraq war.

According to current and former U.S. intelligence officials familiar with the matter, the information that surfaced this summer included intercepted conversations of Iranian officials discussing the country's nuclear weapons program, as well as a journal from an Iranian source that documented decisions to shut it down.

"When we first got some of this stuff, the fact that we got it was exciting," said a senior U.S. intelligence official, who spoke on condition of anonymity because of the classified nature of the subject. He said the information was obtained as part of a stepped-up effort targeting Iran that President Bush had ordered in 2005, but the problem with it "was digesting it to know what we had."

The information triggered a cascade of recalculations across the 16 agencies in the U.S. intelligence community, the official said. Analysts at the CIA and elsewhere began to revisit classified reports that they had scrutinized repeatedly in recent years. As they did so, officials said, they saw details that added up to the new conclusion.

Taking time to reconsider

Intelligence officials said that process of reevaluation was guided by lessons from the prewar intelligence on Iraq. In the months leading up to the war, the intelligence community in just 19 days put together an estimate that concluded that Iraq had stockpiles of chemical and biological weapons. They didn't take the usual time to challenge their assumptions or sources, which later proved to be off-base.

Delivery of the report on Iran was delayed at least three times, according to congressional officials, and the document's authors spent more than a year producing it. Within the last three months, officials said, the CIA used "red teams" -- groups assembled to take opposite or contrarian views -- to challenge the assumptions in the report and scrutinize its claims.

Mindful of criticism of the intelligence on Iraq, officials took steps to insulate the process from political influence.

Intelligence experts praised the rigor of the approach, even as some of them questioned the outcome.

Philip Zelikow, a former senior aide to Secretary of State Condoleezza Rice, is an advocate of diplomacy with Iran, but he said the report understates the threat. The wording of the document "appears to be a reaction to the wording of past estimates," Zelikow said, calling it the latest example of a "pendulum of analytic momentum that swings between highlighting risks and understating risks."

Mark Lowenthal, a former senior CIA official, said, "I think a lot of people are saying, 'Oh good, they learned their lessons from Iraq.' But I'm not sure that's the right answer."

The fact that the community reversed course in the span of two years "should give us a better understanding of what happened in Iraq. It underscores to me that this is tough stuff to do," he said.

The new intelligence was considered compelling enough to call it to Bush's attention in August. In a news conference at the White House on Tuesday, Bush said that the nation's intelligence director, J. Michael McConnell, "came in and said, 'We have some new information.' "

Bush said that McConnell did not provide details. "He didn't tell me what the information was," Bush said. "He did tell me it was going to take awhile to analyze."

The decision to hold those details back has come under question because Bush and others in the administration continued in the succeeding months to use heated rhetoric to warn of the dangers posed by a nuclear-armed Iran. In October, Bush described that scenario as potentially pointing to World War III.

But U.S. intelligence officials said they felt compelled to employ that level of caution in part because of the searing experience surrounding the war in Iraq.