By Paul Reynolds
World affairs correspondent, BBC News website
The startling change in the US intelligence assessment about Iran's nuclear programme is the result of a much more vigorous system introduced since the debacle over Iraq's non-existent weapons of mass destruction.
Iran's Natanz enrichment plant
The National Intelligence Estimate (NIE) on Iran this week concluded that Iran probably did once have a plan to develop a nuclear bomb but halted this in 2003 and had not restarted by mid-2007. There was an important proviso: "We do not know whether it currently intends to develop nuclear weapons."
The assessment is based on a wide range of intelligence sources. These include interceptions of communications, possibly information from defectors, technical information from the International Atomic Energy Agency (the UN body inspecting Iran's declared nuclear facilities) and even an analysis of TV footage from inside Iran's enrichment plant.
Update: according to the New York Times, a key element was the acquisition of notes about meetings between Iranian officials during which military officers complained at the programme being shut down.
This resulted in a detailed exercise to decide whether the information was genuine.
The examination took place in a new atmosphere of scepticism. This differed from the attitude during the Iraq days which tended to think the worst of Iraq's intentions.
The NIE is drawn up by the National Intelligence Council (NIC), formed in 1973. It consults all the US intelligence agencies, which number no fewer than 16, ranging from the CIA to the Treasury Department.
In its introduction to the report on Iran, the NIC says that in the last 18 months it has created new procedures under which the intelligence agencies "are now required to submit formal
assessments that highlight the strengths, weaknesses, and overall credibility of their
sources used in developing the critical judgments of the NIE".
In addition, it says it has "applied more rigorous standards... We have made a concerted
effort to not only highlight differences among agencies but to explain the reasons for
such differences and to prominently display them."
The weakness of consensus
The aim is to avoid the weakness of all systems where co-ordination is the requirement. Co-ordination can lead to consensus and consensus can hide doubts and flaws.
The same weakness was evident in the British system of having a Joint Intelligence Committee, which came to a similarly wrong conclusion over Iraq despite doubts in the system lower down.
One example from Iraq concerned the source known as Curveball, who was given undue prominence, especially in the assessment of so-called mobile chemical weapons vehicles.
The system has also been strengthened by the creation of an overarching Director of National Intelligence (DNI), to whom the NIC reports. It was created after the failures to predict and interdict the attacks of 9/11.
Again the danger of creating yet another post is that intelligence gets reduced in a longer and longer process of examination into the lowest common denominator.
New people at the top
The new thinking is partly due to the two people at the top of these offices.
The DNI is Admiral Mike McConnell, who once directed the National Security Agency, the American electronic eavesdropper and who came out of retirement to do this job.
He told the New York Times in an interview in August: "My job is to speak truth to power."
The other key figure is the head of the NIC Thomas Fingar, who has held a number of senior posts in intelligence in the State Department.
The critics have their doubts
Not that everyone is impressed. President Bush himself picked up the doubts inherent in the NIE and said that Iran remained a danger.
The Israelis have already rejected the NIE finding, claiming that Iran has re-started its nuclear weapons effort.
And critics in the United States are attacking the authors of the report, Mr Fingar among them. The Wall Street Journal said he was one of three "hyper-partisan anti-Bush officials" behind the NIE.
The Carnegie Endowment's Vice President George Perkovich also points out: "The NIE does not say that there should be any confidence that Iran's nuclear program is solely for peaceful purposes, as required under the NPT (the Nuclear Non-Proliferation Treaty)."
And of course there is no guarantee that this report will be any more accurate than the 2005 one. The history of intelligence is signposted by mistakes.