WASHINGTON (CNN) - The Treasury Department said Monday it is looking into how a government document from the very early days of the Bush administration -- marked "secret" and outlining plans for a post-Saddam Iraq -- became part of a CBS "60 Minutes" broadcast Sunday night.

"Based on the '60 Minutes' segment aired Sunday evening, there was a document that was shown that appeared to be classified," said Treasury Department spokesman Rob Nichols. "It was for that reason that it was referred to the U.S. inspector general's office."

Ex-Treasury Secretary Paul O'Neill

Ousted Treasury Secretary Paul O'Neill, now an outspoken critic of the Bush administration, was a guest on the program, along with Ron Suskind, the author of a book for which O'Neill was the primary source. O'Neill said on the program that the administration was preparing plans to move against Iraq "from the very beginning."

Suskind told CNN he had no access to secret documents and O'Neill never improperly took classified papers after leaving the Administration.

"I am certain O'Neill never had it," said Suskind, author of "The Price of Loyalty", referring to a National Security Council document on post-Sadaam Iraq.

The Treasury Department is looking into how a government document that may be classified was shown in an interview with ex-Treasury Secretary Paul O'Neill on 60 Minutes. CNNfn's Louise Schiavone reports. Play video (Real or Windows Media)

Suskind said O'Neill gained proper authorization to all 19,000 Government documents used as sources for the book. "He got the documents from lawyers at the Treasury Department when he made a request after he left," said Suskind.

Paul O'Neill was traveling Monday afternoon and did not return a phone call from CNN.

The decision to refer the matter to the inspector general was made at a Monday morning Treasury Department meeting involving senior staff and department attorneys, Nichols said, and was made without the initiation or consultation of the White House.

There is a precedent for former Cabinet members to take information -- such as schedules, letters, press releases and speeches -- with them as they leave, but it would be illegal to take classified information.

The request for the investigation came as O'Neill's comments critical of the Bush administration sparked a fury of controversy in Washington. O'Neill clashed with the president on deficit spending and tax cuts, which ultimately led to O'Neill's departure.

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Asked if seeking the probe may look vindictive, Nichols said, "We don't view it in that way," according to Reuters news agency.

In the book, O'Neill describes a disengaged President Bush who appeared determined to bring the United States into a conflict with Iraq for the purpose of ousting Saddam Hussein.

Asked about the assertions Monday in Mexico, President Bush defended his decision to go to war with Iraq and disputed O'Neill's comments.

In the upcoming book, O'Neill compares Bush's presence at Cabinet meetings to "a blind man in a room full of deaf people."

O'Neill also maintains that his advice to Vice President Dick Cheney about steel tariffs and tax cuts was ignored, largely due to political considerations, according to excerpts from the book printed in Monday's Wall Street Journal.

O'Neill, who had served in the Ford administration and also as CEO of Alcoa Inc., a big aluminum producer, had argued that tax cuts would do serious damage to the federal budget and that tariffs would do little to help domestic steelmakers in the long run.

O'Neill's account of a disengaged President Bush eyeing war with Iraq from his first days in office drew jeers from White House officials, but Democrats said O'Neill's story shows Bush misled Americans about the road to war.

Commerce Secretary Don Evans, who was Bush's 2000 campaign chairman, said Bush asks tough questions and encourages debate in Cabinet meetings.

"He likes to see debate," Evans said. "He thinks it's very healthy, very constructive for the process. Oftentimes, he has to make the deciding decision when he has his advisers on both sides of the same subject."

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Rep. Dick Gephardt, a Democratic presidential contender who supported the war, said at a campaign stop in Iowa: "It's a worrisome fact, and we need to look into it and find out what really went on."

Suskind's book is scheduled for publication Tuesday. O'Neill gave Suskind 19,000 internal documents and took no money for his role in the book.

A CBS News spokesperson said, "We did not show any secret documents. We do not have secret documents. We showed the cover sheet that alluded to such documents."

Former Sen. Bob Dole, the Republicans' presidential nominee in 1996, said he wouldn't suggest O'Neill was "bitter," but "certainly very critical."

"I mean, there's always somebody in somebody's administration who jumps out early, sells a book, and goes after the guy who hired him," Dole told CNN. "I don't know if that's good. It may be good business, it's not good politics."