Former chief US weapons hunter David Kay said in an interview on Friday that the intelligence services' reliance on information from Iraqi defectors had led the West to misjudge Saddam Hussein.
Kay told Britain's Daily Telegraph newspaper that different agencies all but competed with each other over the intelligence without realising that much of it came from the same source.
The US failed to appreciate that all the agencies effectively used Ahmad Chalabi, leader of the then-exiled Iraqi National Congress group, as the only - and Kay believes flawed - source.
"There was no adult leadership at the top forcing a realistic assessment," said Kay, who resigned his post in January saying he now believed there were no weapons of mass destruction in Iraq.
Chalabi, said Kay, succeeded in spreading his message: "He did not just funnel people to the United States, he funnelled people to other governments."
Chalabi, who maintains that weapons of mass destruction will be found in Iraq, is now a member of the US-appointed Iraqi Governing Council. He has said the CIA should have done a better job analysing information from defectors he steered their way.
Kay advocated a wholesale reform of the use of intelligence: "Policymakers tend to read the front pages of documents, the summaries never have the caveats on the first page."
His successor continues to determine why weapons of mass destruction, the main reason given by US President George W. Bush for the war against Iraq, have not been found.
Kay, who is still a supporter of the war, also cited as problems the lack of agents on the ground in Iraq and a failure to appreciate the true nature of Saddam's regime.
Western nations, he said, had failed to understand that Saddam had become a classic delusional tyrant, spending most of his money on palaces, and bluffing about his weaponry to keep the world at bay.