But Rumsfeld on Wednesday held out the possibility that weapons of mass destruction in Iraq would eventually be found - despite last month's conclusion by the group's departing leader, David Kay, that no stockpiles of such arms existed in Iraq when it was invaded last March.
Testifying before committees of the Senate and House of Representatives, Rumsfeld also defended the war despite the doubts over the presence of these arms.
"I'm convinced that the president of the United States did the right thing in Iraq. Let there be no doubt," Rumsfeld told the Senate Armed Services Committee. "The world is a safer place today and the Iraqi people far better off for that action."
The existence of chemical and biological weapons and a reconstituted nuclear arms programme was cited by President George Bush, Rumsfeld and other US officials as a major reason for invading Iraq and overthrowing President Saddam Hussein.
Publicly addressing Kay's comments for the first time, Rumsfeld acknowledged that Iraq may not have possessed weapons at the start of the war.
"I suppose that's possible, but not likely," Rumsfeld testified. He put forward several competing theories to explain why such arms had not yet been found in Iraq:
- such weapons may not have existed at the start of the war
- Iraq had such weapons but they were "transferred in whole or in part to one or more other countries"
- such weapons existed but were "dispersed and hidden throughout Iraq"
- these weapons were "destroyed at some moment prior" to the war
- Iraq possessed small quantities of biological or chemical agents, and had "a surge capability for a rapid build-up - and that we may eventually find it in the months ahead"
- possession of these weapons was "a charade by the Iraqis" either with Saddam fooling everyone into thinking he had them, or "his own people" tricking him into believing he had capabilities that did not exist.
Rumsfeld indicated that he thought weapons might still be hidden, but did not state which, if any, of the other theories he believed.
"It took us 10 months to find Saddam Hussein. The reality is that the hole he was found hiding in was large enough to hold enough biological weapons to kill thousands of human beings," Rumsfeld said.
Saddam 'may have exaggerated'
his weapons capabilities
Saddam was reportedly captured in a hole in the ground at a farm near Tikrit on 13 December 2003.
Rumsfeld faced sharp questioning by Democrats.
Senator Edward Kennedy of Massachusetts confronted Rumsfeld with his own statement before the war that "we know where they are," referring to Iraqi weapons of mass destruction. Rumsfeld said he meant to say suspected weapons sites.
"I'm sure from time to time I say something that, in retrospect, I wish I hadn't," Rumsfeld said.
Rumsfeld said the search for weapons of mass destruction under Kay's successor "is some distance from completion", adding that "when that work is complete, we will know more".