"Not only have I been studying this for many, many years, but, as I prepared that statement, I worked very closely with the director of central intelligence, George Tenet," Powell told the "Fox News Sunday" program.

"That statement was vetted thoroughly by all of the analysts who are responsible for this account. We spent four days and nights out at the CIA, making sure that whatever I said was supported by our intelligence holdings.

"Because it wasn't the president's credibility and my credibility in line, it was the credibility of the United States of America," he said.

Washington has come under mounting criticism over the failure of US-led troops in Iraq to find the weapons, and US lawmakers have launched an investigation into whether the White House distorted intelligence to justify the war.

"There can be no question there were weapons before the war," Powell insisted.

"They have used chemical weapons. They have admitted that they had biological weapons. And they never accounted for all that they had or what they might or might not have done with it."

"Now, we have to do the intensive search that is ahead of us, and the Iraq survey group will be adding that. And I'm sure more evidence and more proof will come forward as we go down this road."

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