In an interview with National Public Radio on Sunday, the ex-inspector said he led the search to find truth and weapons – but found neither.
"The weapons did not exist, we've got to deal with that … and understand why."
Asked if he thought President George Bush owed the American public an explanation for the failure to find banned WMD, he added: "The intelligence community owes the president rather than the president owing the American people."
Kay stepped down on Friday as leader of the Iraq Survey Group, which 10 months after the US and British invasion of Iraq has yet to find any chemical or biological weapons.
The failure has become a major embarrassment for Washington after it made WMD the central element of its case for war against Iraq.
Kay also expanded on remarks he made earlier to Britain's Sunday Telegraph newspaper, on the possibility that Iraq had sent some of its weapons to Syria before the war.
"There's ample evidence of movement to Syria before the war," he said. "There's satellite photography, there are reports on the ground, of a constant stream of trucks, cars, rail traffic across the border. We simply don't know what was moved."
"The intelligence community owes the president [an explanation] rather than the president owing the American people."
Former chief weapons inspector
"There's very little you can do in Iraq to determine what was moved. The real answers to that are in Syria, and the Syrian government has shown absolutely no interest in helping us resolve this issue," he said.
Kay's remarks also cast doubt on claims contained in a British government dossier which said in September 2002 that Iraq could deploy chemical or biological weapons in just 45 minutes.
"After the war and with the inspection effort that we have carried out now for nine months, I think we all agree that there were not large amounts of weapons available for imminent action," he said.
"That's not the same thing as saying it was not a serious, imminent threat that you're not willing to run for the nation. That is a political judgment, not a technical judgment."
US Secretary of State Colin Powell said on Saturday it was an "open question" as to whether Iraq still had such weapons, but he argued that pre-war intelligence was correct about former president Saddam Hussein's intention to develop them.