The former senior US weapons inspector, David Kay, has said intelligence that Iraq possessed stockpiles of chemical and biological weapons was false.
"We were all wrong," said Kay
Mr Kay, who resigned last week, told a Senate committee in Washington that failures had become apparent in US intelligence-gathering capability.
He denied that US intelligence had been distorted by government pressure in the run-up to last year's US-led war.
"It turns out we were all wrong," he said, "and that is most disturbing."
In his evidence to the Senate armed services committee, Mr Kay repeated his assertion that there were no illegal weapons stockpiles in Iraq.
"I believe that the effort that has been directed to this point
has been sufficiently intense that it is highly unlikely that there were large stockpiles of deployed, militarised chemical weapons there," he said.
He said that the failure to find such stockpiles exposed weaknesses in American intelligence-gathering.
"It's quite clear we need capabilities that we do not have with regard to intelligence," he said.
However, Mr Kay rejected suggestions by opposition Democrats that intelligence had been manipulated in order to justify the decision to go to war.
"The fact that it wasn't tells me we have a much more fundamental problem of understanding what went wrong. And we've got to figure out what was there."
He said he would favour an independent inquiry into the intelligence failures.
Although his team's quest has so far proved fruitless, Mr Kay said the hunt for Iraq's weapons of mass destruction should continue.
He acknowledged there was a "theoretical possibility" that weapons could still be found and said there would always be an "unresolvable ambiguity" about Iraqi weapons programmes.
Mr Kay said much evidence had been lost in the immediate aftermath of the war because of the looting and chaos that had ensued in Iraq.
He said some Iraqis probably took advantage of the instability to destroy any evidence of weapons programmes.
He added: "I think the world is far safer with the disappearance and the removal of Saddam Hussein."