Bush administration officials have been accused of misrepresenting the threat from Iraqi weapons of mass destruction.
US troops have been searching for WMD in Iraq
The accusation comes in a report from the influential left-of-centre Carnegie Endowment for International Peace, which opposed the war in Iraq.
It also says there was no evidence for the claim that Saddam Hussein would give such weapons to terrorists.
The US and UK cited concerns over weapons of mass destruction to justify the invasion of Iraq last March.
Meanwhile, a 400-strong US team of weapons disposal experts is being withdrawn from Iraq.
US Government officials confirmed reports in the New York Times that the mission was being wrapped up, but stressed that the team had finished its work.
The newspaper reported that 400 - out of a team of 1,400 - had been assigned to search for depots for missile launchers and other equipment that might be used in conjunction with weapons of mass destruction.
The Carnegie Endowment said it had studied hundreds of documents and interviewed dozens of specialists for its report WMD: Evidence and Implications.
The report says there was "no convincing evidence" that Iraq had reconstituted its nuclear programme.
There was greater uncertainty about its biological weapons, it continues, but the threat related to what could be developed in future rather than what Iraq actually had.
The report says it was unlikely Iraq could have destroyed, hidden or moved large amounts of chemical and biological weapons without the United States detecting some sign of activity.
And it adds: "There was no evidence to support the claim that Iraq would have transferred WMD to al-Qaeda and much evidence to counter it."
The study concludes that while the long-term threat from Iraq could not be ignored, it was being contained by a combination of UN weapons inspections, international sanctions and limited US-led military action.
"Administration officials systematically misrepresented the threat from Iraq's WMD and ballistic missile programmes," it contends.