Special Counsel Patrick Fitzgerald. the special prosecutor in the criminal case against Vice President Dick Cheney's former chief of staff said in a Jan. 23 letter that not all e-mail was archived in 2003, the year the Bush administration exposed the identity of undercover CIA officer Valerie Plame.

Lawyers for defendant I. Lewis "Scooter" Libby this week accused prosecutors of withholding evidence the Libby camp says it needs to mount a defense.

"We are aware of no evidence pertinent to the charges against defendant Libby which has been destroyed," Fitzgerald wrote in a letter to the defense team.

But the prosecutor added: "In an abundance of caution, we advise you that we have learned that not all e-mail of the Office of Vice President and the Executive Office of the President for certain time periods in 2003 was preserved through the normal archiving process on the White House computer system." His letter was an exhibit attached to Libby's demand for more information from the prosecution.

Lea Anne McBride, a spokeswoman for Cheney, said the vice president's office is cooperating fully with the investigation, and referred questions to Fitzgerald's office.

Libby is charged with five counts of perjury, obstruction and lying to the FBI regarding how he learned of Plame's identity and what he did with the information.

The Presidential Records Act, passed by Congress in 1978, made it clear that records generated in the conduct of official duties did not belong to the president or vice president, but were the property of the government.

The National Archives takes custody of the records when the president leaves office.

"Bottom line: Accidents happen and there could be a benign explanation, but this is highly irregular and invites suspicion," said Steve Aftergood, director of the Federation of American Scientists government secrecy project.

"A particular subset of records sought in a controversial prosecution have gone missing," Aftergood said. "I think what is needed is for the national archivist to ascertain what went wrong and how to ensure it won't happen again."