Vice President Dick Cheney could be called to testify in the perjury case against his former chief of staff, a special prosecutor said in a court filing Wednesday.

Vice President Dick Cheney delivers a speech at a luncheon held for congressional candidate Brian Bilbray at the Sheraton Hotel and Marina Tuesday, May 23, 2006 in San Diego, Calif. [AP]

Special Counsel Patrick Fitzgerald suggested Cheney would be a logical government witness because he could authenticate notes he jotted on a July 6, 2003, New York Times opinion piece by a former U.S. ambassador critical of the Iraq war.

Fitzgerald said Cheney's "state of mind" is "directly relevant" to whether I. Lewis "Scooter" Libby, the vice president's former top aide, lied to FBI agents and a federal grand jury about how he learned about CIA officer Valerie Plame's identity and what he subsequently told reporters.

Libby "shared the interests of his superior and was subject to his direction," the prosecutor wrote. "Therefore, the state of mind of the vice president as communicated to (the) defendant is directly relevant to the issue of whether (the) defendant knowingly made false statements to federal agents and the grand jury regarding when and how he learned about (Plame's) employment and what he said to reporters regarding this issue."

In the Times article, former U.S. Ambassador Joseph Wilson accused the Bush administration of twisting intelligence on Iraq to justify going to war. In 2002, the CIA sent Wilson to Niger to determine whether Iraq tried to buy uranium yellowcake from Niger to build a nuclear weapon. Wilson discounted the reports. But the allegation wound up in President Bush's 2003 State of the Union address.

Cheney wrote on the article, "Have they done this sort of thing before? Send an ambassador to answer a question? Do we ordinarily send people out pro bono to work for us? Or did his wife send him on a junket?"

Libby told the agents and the grand jury that he believed he had learned from reporters that Plame is married to Wilson and had forgotten that Cheney had told him that in the weeks before Wilson's article was published.

In his grand jury testimony, Libby said Cheney was so upset about Wilson's allegations that they discussed them daily after the article appeared. "He was very keen to get the truth out," Libby testified, quoting Cheney as saying, "Let's get everything out."

Cheney viewed Wilson's allegations as a personal attack because the article suggested that the vice president knew that Wilson had discounted old reports that Iraq had tried to buy uranium yellowcake from Niger to build a nuclear weapon.

Eight days after Wilson's article, conservative syndicated columnist Robert Novak identified Plame and suggested that she had played a role in the CIA's decision to send Wilson to Niger.

Fitzgerald contends that Plame's status as a CIA officer was classified and that Libby was told that disclosing her identity could pose a danger.

The prosecutor wants to use Cheney's notes on the Wilson article to corroborate other evidence he has that Libby lied about outing Plame to reporters.

In a filing last week, Libby's lawyers said Fitzgerald would not call Cheney as a witness and would have a hard time getting the vice president's notes admitted into evidence.

"Contrary to defendant's assertion, the government has not represented that it does not intend to call the vice president as a witness at trial," Fitzgerald wrote. "To the best of government's counsel's recollection, the government has not commented on whether it intends to call the vice president as a witness."