The US chief weapons inspector, Charles Duelfer, has said inquiries into weapons of mass destruction in Iraq have "gone as far as feasible".
Duelfer said Saddam Hussein had created a pool of weapons experts
Mr Duelfer also said an official transfer of WMDs to Syria ahead of the Iraq war was not likely.
The CIA adviser reported last year that neither expected stockpiles of chemical or biological weapons, nor evidence of recent production had been found.
However, he did say Saddam Hussein had wanted to restart WMD programmes.
"After more than 18 months, the WMD investigation and debriefing of the WMD-related detainees has been exhausted," Mr Duelfer, head of the Iraq Survey Group (ISG), wrote in a 92-page addendum to the report issued in October.
However, Mr Duelfer warned that Iraq's original weapons programme had created a pool of experts whose skills could be sought by other countries or terrorist groups, and that while this risk was presently very small, it should not be ignored.
IRAQ SURVEY GROUP
Set up in May 2003
First leader, David Kay, quit in Jan 2004 stating WMD would not be found in Iraq
New head, Charles Duelfer appointed by CIA
1,200 experts from the US, Britain and Australia
HQ in Washington, offices in Baghdad and Qatar
"The use of a single even an ineffectual chemical weapon would likely cause more terror than deadlier conventional explosives," the supplementary report warned.
Mr Duelfer said that while the ISG believed that it was unlikely that WMD material had been officially moved to Syria in the run up to the war, it was "unable to rule out unofficial movement of limited WMD-related materials".
The US and Britain used allegations that Iraq possessed WMDs as the primary justification for invading Iraq in 2003.