US Secretary of State Colin Powell has admitted that evidence he submitted to the United Nations to justify war on Iraq may have been wrong.
There is still no sign of WMD in Iraq
In February last year he told the UN Security Council that Iraq had developed mobile laboratories for making biological weapons.
On Friday he conceded that information "appears not to be... that solid".
The claim failed to persuade the Security Council to back the war, but helped sway US public opinion.
Mr Powell said he hoped the commission appointed to investigate pre-war intelligence on Iraq would examine whether the intelligence community was justified in backing the claim.
Doubts have been widely cast on the existence of the mobile labs, not least by the former US chief weapons inspector in Iraq, David Kay, who now says does not know whether Iraq ever had a mobile weapons programme.
No evidence of weapons of mass destruction has emerged in Iraq since the end of the war.
Mr Powell said the US intelligence officers "indicated to me" that the information about the mobile labs was reliable, and "I made sure it was multi-sourced".
"Now, if the sources fell apart we need to find out how
we've gotten ourselves in that position," he said.
"I have discussions with the CIA about it," he said,
without providing further details.
It is the first time Mr Powell has acknowledged key evidence he used to make the case for war may have been wrong, says the BBC's Jannat Jalil in Washington.
Previously, he has only said that he does not know if he would have backed the invasion had he believed Iraq did not possess banned weapons.
This admission by Mr Powell could further hurt the credibility of the Bush administration in what is an election year, our correspondent says.
Mr Powell referred to several intelligence sources on the trailers during his Security Council speech, but at least two have been questioned in recent weeks.
News organisations have reported that US intelligence officials considered one source unreliable even before Mr Powell's speech.
The Los Angeles Times also alleged that another source had been widely discredited and was never even interviewed by US officials.
Mr Powell's admission comes as the US's intelligence record is scrutinised over the attacks on 11 September, 2001.
Mr Powell last month appeared before a commission looking into the attacks, and denied that the Bush administration ignored the threat from al-Qaeda and Osama Bin Laden.
He was questioned following allegations from ex-White House counter-terrorism aide Richard Clarke that Mr Bush and his colleagues were so preoccupied with launching a war on Iraq, that they missed the growing threat from al-Qaeda.