Special Prosecutor Patrick Fitzgerald is asking a federal judge not to force him to explain during the impending trial of a former White House aide why nobody was charged with leaking the identity of a CIA operative.

Vice President Dick Cheney's former chief of staff, I. Lewis Libby, faces trial in January for obstruction and perjury, the only person charged in the case. His supporters have accused Fitzgerald of singling out Libby while not charging the source of the leak, former Deputy Secretary of State Richard Armitage.

In court documents filed Monday, Fitzgerald said he should not have to deal with that question at Libby's trial. While never speaking of Armitage by name, Fitzgerald said it would be irrelevant to discuss other officials.

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"If Mr. X was investigated for leaking classified information, the government's decision not to charge Mr. X should have nothing to do with the jury's role as the finder of fact in Libby's case," Fitzgerald wrote.

If jurors should disagree with his decision not to charge anyone else, they might be less likely to convict Libby, Fitzgerald wrote.

Armitage told prosecutors he inadvertently revealed Plame's job to syndicated columnist Robert Novak in July 2003. Novak's story ran as Plame's husband, Joseph Wilson, criticized the Bush administration's prewar intelligence on Iraq.

Libby was among several Bush administration officials questioned about the leak, and prosecutors said he lied about his conversations with reporters.

Defense attorneys have sought information regarding Armitage's role in the leak, and their pleas have been rejected.

Fitzgerald's court filing was an attempt to prevent Libby's attorneys from making the argument that Libby had no reason to lie because, if he had leaked classified information, prosecutors would have charged him with it.

"The government's decision not to charge a crime is not a declaration as to the legality or propriety of the defendant's conduct," Fitzgerald wrote.

Libby's defense attorneys are expected to respond to Fitzgerald's motion.

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