The 1700-member Iraq Survey Team, which scoured Iraq for the weapons, also said in a report released late on Monday that it found no Iraqi officials with direct knowledge of a transfer of weapons of mass destruction allegedly developed by former President Saddam Hussein.
President George Bush and other US officials cited a grave threat posed by Iraq's chemical and biological weapons and Baghdad's efforts to acquire a nuclear arms capability as a justification for war.
No such weapons were found but US officials said it was possible Hussein sent them to Syria for safekeeping.
The report is the final addendum to the investigators' September report that concluded prewar Iraq had no WMD stockpiles of biological and chemical weapons and that its nuclear programme had decayed before the US-led invasion.
US report in September said Iraq
had no WMD stockpiles
The Iraq Survey Group, led by CIA special adviser Charles Duelfer, wrapped up its physical searches for weapons of mass destruction last December.
The new report posted on the CIA website said: "Based on evidence available ... it is unlikely that an official transfer of WMD material from Iraq to Syria took place. However ISG was unable to rule out unofficial movement of limited WMD-related materials."
It said investigators found no senior policy, programme or intelligence officials who admitted any direct knowledge of such movement of WMD.
"Indeed, they uniformly denied any knowledge of residual WMD that have been secreted to Syria," the report said.
"After more than 18 months, the WMD investigation and debriefing on the WMD-related detainees have been exhausted"
The report said the WMD investigation had gone as far as possible and there was no reason to continue holding many of the Iraqis who had been detained in the process.
"After more than 18 months, the WMD investigation and debriefing on the WMD-related detainees have been exhausted," the report said.
It noted there was a risk some Iraqi scientists might share their skills with anti-US fighters. The report added the pool of scientists who still possessed potentially dangerous expertise was shrinking.