In a direct challenge to the Bush administration, which says its invasion of Iraq was justified by the presence of illicit arms, David Kay said on Friday that it is unlikely anything significant will be found.
"I don't think they existed," Kay said. "What everyone was talking about is stockpiles produced after the end of the last (1991) Gulf War, and I don't think there was a large-scale production programme in the nineties," he said.
The CIA announced earlier that former UN weapons inspector Charles Duelfer, who has previously expressed doubts that unconventional weapons would be found, would succeed Kay as Washington's chief arms hunter.
Kay said he believes most of what was going to be found in the search for weapons of mass destruction in Iraq has been found, and the hunt would become more difficult once America returns control of the country to the Iraqis.
Iraqi weapons threat
The United States went to war against Baghdad last year citing a threat from Iraq's weapons of mass destruction. To date, no banned arms have been found.
In his annual State of the Union on Tuesday, President George Bush insisted former Iraqi leader Saddam Hussein had actively pursued dangerous programmes right up to the start of the US attack in March.
"I don't think they (Iraqi WMDs) existed. What everyone was talking about is stockpiles produced after the end of the last (1991) Gulf War, and I don't think there was a large-scale production programme in the nineties"
Former Iraq Survey Group leader
Citing a report to Congress in October, Bush said Kay had found "dozens of weapons of mass destruction-related programme activities and significant amounts of equipment that Iraq concealed from the United Nations".
"Had we failed to act," Bush said, "the dictator's weapons of mass destruction programmes would continue to this day."
And on Wednesday, Vice President Dick Cheney said the United States had not given up on finding unconventional weapons in Iraq. "The jury is still out," he said in a radio interview.
Iraqi resistance attacks
Kay said he left the post due to a "complex set of issues".
It related in part to a reduction in the resource and a change in focus of the Iraq Survey Group, which is in charge of the weapons hunt.
ISG analysts were diverted from hunting for weapons of mass destruction to helping in the fight against Iraqi resistance attacks, Kay said.
"When I had started out I had made it a condition that ISG be exclusively focused on WMD, that's no longer so," he said.
"We're not going to find much after June. Once the Iraqis take complete control of the government it is just almost impossible to operate in the way that we operate."
"I think we have found probably 85% of what we're going to find. I think the best evidence is that they did not resume large-scale production and that's what we're really talking about."
Kay said he was going back to the private sector.
Bush's main justification for war
was the threat of Iraqi weapons
Former UN inspector
In a statement announcing Kay's departure, CIA Director George Tenet praised Kay for his "extraordinary service under dangerous and difficult circumstances".
Meanwhile, Duelfer, 51, is a former deputy executive chairman of the UN Special Commission that was responsible for dismantling Iraq's WMDs.
He told NBC television earlier this month. "I think that Mr Kay and his team have looked very hard. I think the reason that they haven't found them is they're probably not there."
But in a statement included in the CIA announcement, Duelfer, who will be based in Iraq and as CIA special adviser to direct the WMD search, said he was keeping an open mind.
"I'm... absolutely committed to following the evidence wherever it takes us," he said.