Federal prosecutors investigating the leak of former CIA operative Valerie Plame's identity have asked several witnesses in the case whether they read a State Department memorandum mentioning her that circulated inside the Bush administration in the days before she was publicly named, according to people familiar with the testimony.

FBI agents showed the State Department memo to several witnesses during the interviews over the past two years, according to lawyers in the case, in an effort to determine whether it was the source of information about Plame's role at the CIA. A key mystery in the leak case is how senior administration officials first learned of Plame's identity and her relationship to a key critic of President Bush's Iraq policy, before her name appeared in news reports.

Lawyers familiar with the testimony of White House senior adviser Karl Rove said he has admitted discussing Plame, though not by name, but said he learned of her role from a reporter. Several legal sources said the prosecution has shown strong interest in the State Department memo, which circulated on Air Force One during the Africa trip -- just days before Plame's name was made public in a column by Robert D. Novak.

Prosecutors are investigating whether administration officials leaked Plame's name to retaliate against her husband, Joseph C. Wilson IV, an ex-diplomat who had accused Bush of twisting intelligence to justify the Iraq war. Wilson, on a mission authorized by the CIA, went to Niger to investigate whether Iraq was seeking uranium for nuclear bombs. He reported that there was no evidence to support that suspicion.

Federal prosecutors are investigating whether then-Secretary of State Colin L. Powell, who was on the Africa trip with Bush, carried with him a memo containing information on Plame, as well as other intelligence about allegations made by Wilson.

According to people involved in the case, prosecutors believe that a printout of memo was in the front of Air Force One during a July 7-12 trip Bush took to Africa, but investigators are unsure who reviewed or obtained copies of it. One of the earliest moves by special prosecutor Patrick J. Fitzgerald, signaling his aggressive stance, was to get the grand jury to subpoena Air Force One phone logs from the trip, the sources said. Newsweek reported in August 2004 that Powell's testimony before the grand jury focused, in part, on the memo.

The memo "identifies her as having selected or recommended her husband" for the Niger assignment, according to a person who has seen it. Administration officials circulated this information as a way of discrediting the reliability of Wilson's charges.

Lawyers involved in the probe said prosecutors are interested in whether anyone called back to Washington to talk about information in the memo. Prosecutors have asked numerous questions about then-White House press secretary Ari Fleischer, who was on the trip and aboard Air Force One, according to the lawyers. Fleischer has declined to comment.

Rove said of the memo that he "had never seen it, had never heard about it and had never heard anybody else talk about it," according to a lawyer familiar with his testimony. Rove's attorney, Robert Luskin, said he can say "categorically" that Rove did not obtain any information about Plame from any confidential source, such as a classified document.

The memo was first reported by the New York Times, but several sources had described its content to The Washington Post in interviews this week.