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.

"Back in 2002, one of the knocks on the process at the time was that information was not vetted by analysts and was being rushed into the Oval Office," said the senior U.S. intelligence official.

That experience showed, the official said, that bringing unvetted intelligence to senior officials could backfire.

This time, even as they vetted the new intelligence and launched into major revisions of the estimate on Iran's nuclear program, intelligence officials said, they deliberately shielded analysts from administration officials and policymakers.

"There was a lot of criticism, justifiably or not, of policymakers shaping the Iraq WMD," the senior intelligence official said, referring to weapons of mass destruction. "This was the intelligence community, without political interference, looking at the facts, reaching its judgments."

As a result, it wasn't until about two weeks ago that Vice President Dick Cheney, national security advisor Stephen Hadley and other high-level officials received initial briefs on the pending findings. The White House has not clarified when Bush was first told that it appeared Iran had halted its nuclear weapons program in 2003.

Officials declined to discuss the new intelligence publicly, citing the need to protect sources and methods. But current and former officials provided basic descriptions of it, saying that the intercepts were of a series of conversations involving an Iranian official "complaining in 2007 about the suspension of the military program in 2003."

Another was described as a journal or diary by an Iranian "involved in the management of the military program," a former official said. One source said the diary came from a defector, but other intelligence officials said that was not the case.

Although the new information came from only a limited number of sources, officials said it was considered reliable. "I'm not saying there's a wide range of sources," said an official familiar with the intelligence. "But the volume we got is significant."

Officials also emphasized that confidence in the information came from comparing it with existing data from other sources. That wasn't the case in the run-up to the war in Iraq, when a defector known as Curveball provided wildly inaccurate claims about his country's weapons of mass destruction.

Strict reevaluation

David Albright, director of the nonprofit Institute for Science and International Security, said the shift in emphasis resulted as much from a strict reevaluation of old intelligence as from any discovery of dramatic new evidence.

"They went back and looked at the old information," said Albright, who noted that he had discussed the matter with the nation's No. 2 intelligence official, Donald M. Kerr. "Then there was a huge fight, because the people who came up with the first view really defended it. But they couldn't justify it."

In the report released Monday, the intelligence community concluded "with high confidence that in fall 2003, Tehran halted its nuclear weapons program." But the document went on to note that Iran "at a minimum is keeping open the option to develop nuclear weapons" and could resume work anytime.

The magnitude of that reversal and the public manner in which it was revealed are rare, if not unprecedented, in the history of U.S. intelligence. And the release of the report has revived debate over the quality and credibility of U.S. intelligence.

"I think that it will reinforce critics' opinion that they don't know what they're doing," said Daniel Byman, director of Georgetown University's security studies program, and a former CIA analyst. "They're saying basically they got their earlier call wrong, which will allow people to say, 'Why should we believe you now?' "

On the other hand, Byman said, "they can hardly be accused of being politicized, because they're directly going against the Bush administration's statements on the issue."

Already there is evidence of skepticism about the new report. Sen. Christopher S. Bond of Missouri, the ranking Republican on the Senate Intelligence Committee, said he wouldn't be able to determine the quality of the intelligence estimate until the committee had had a chance to review the underlying intelligence.

Some U.S. allies, particularly Israel, which has previously shared intelligence on Iran, also criticized the report. Israeli Defense Minister Ehud Barak disputed its findings, saying Israel believes Iran is still trying to develop a nuclear weapon.

Israeli Prime Minister Ehud Olmert said that he, Barak and other Israeli officials were briefed in advance on the U.S. intelligence assessment when they visited Washington last week.

greg.miller@latimes.com

Times staff writers Richard Boudreaux in Jerusalem and Bob Drogin in Washington contributed to this report.