More than four months after US President George Bush declared victory in Iraq, Blix said facts presented by Iraq in a 12,000-page document in December 2002 may have been accurate.
"With this long period, I'm inclined to think that the Iraqi statement that they destroyed all the biological and chemical weapons, which they had in the summer of 1991 may well be the truth," Blix told CNN television.
No smoking gun
The retired Swedish diplomat, who headed the United Nations Monitoring, Verification and Inspection Commission for Iraq, said his inspectors had worked in Iraq for three-and-a-half months in late 2002 and early 2003 and "did not find any smoking gun."
Blix said US and British experts had been scouring Iraq for weapons of mass destruction for several months and had the opportunity to interrogate members of the Iraqi establishment in their custody.
"I cannot fail to notice that some of the things that they expected us to see that they have turned out not to be real weapons of mass destruction," said the former chief inspector.
The declaration, submitted on 7 December by the government of then-Iraqi President Saddam Hussein, was quickly dismissed as false and incomplete by the United States and Britain, which accused Baghdad of failing to disarm as required by Security Council Resolution 1441.
These charges were later used by Washington and London to justify their invasion of Iraq in late March.