US Vice-President Dick Cheney could be called to testify in the CIA leak case involving ex-chief of staff Lewis Libby, a US prosecutor said Wednesday.
Lewis Libby is accused of lying to investigators
Special Counsel Patrick Fitzgerald said Mr Cheney's state of mind was "directly relevant" to the case, in a pre-trial document filed to a Washington court.
He said Mr Cheney could give evidence on notes he wrote on a copy of a newspaper article linked to the case.
Mr Libby faces charges in connection with the leak of an agent's identity.
Mr Libby, whose trial is not due to begin till next year, denies five charges of perjury, making false statements and obstruction of justice.
The identity of CIA agent Valerie Plame was leaked in 2003.
Her husband Joseph Wilson, a former diplomat, had criticised US Iraq war policy and wrote the New York Times opinion piece cited by the prosecutor shortly before the leak.
Mr Fitzgerald is investigating whether administration officials broke the law by deliberately disclosing her identity.
Mr Libby is accused of lying to FBI investigators and a grand jury about how and when he learned that Ms Plame was a CIA officer and of lying about disclosing classified information to reporters.
In the court filing - which does not state categorically whether Mr Cheney will testify - Mr Fitzgerald said Mr Libby has acknowledged that he and the vice-president had discussed Mr Wilson's article.
CIA LEAK TIMELINE
6 Jul 2003: Joseph Wilson questions US claims about Iraq nuclear programme
8 Jul: Libby leaks classified information to reporter Judith Miller, but not agent's name, he later testifies
14 Jul: Columnist Robert Novak identifies Wilson's wife as CIA agent
30 Sept: Justice dept launches inquiry into agent's outing
28 Oct 2005: Libby charged with obstruction and perjury
6 Apr 2006: Court papers suggest Bush authorised leak of classified material (not agent's identity)
"Here as defendant has acknowledged, the vice-president communicated to defendant the facts he considered notable, and also directed defendant to get out to the public 'all' the facts in response to the Wilson Op Ed," he wrote in the court filing.
"Contrary to [Mr Libby's] assertion, the government has not represented that it does not intend to call the vice-president as a witness at trial," Mr Fitzgerald said in answering a request by Mr Libby for more documents on the case.
Mr Libby's lawyers had suggested in the request Mr Cheney would be irrelevant as a witness in the trial.
Mr Libby "shared the interests of his superior and was subject to his direction. Therefore, the state of mind of the vice-president as communicated to defendant is directly relevant to the issue of whether defendant knowingly made false statements to federal agents," the prosecutor said in the filing.
Mr Cheney wrote on the article: "Have they done this sort of thing before? Send an ambassador [Joseph Wilson] to answer a question? Do we ordinarily send people out pro bono to work for us? Or did his wife [Valerie Plame] send him on a junket?"
Mr Wilson had been sent to Niger in 2002 to investigate reports that Iraq had bought or had sought to buy uranium there.