Prosecutors told jurors Tuesday that former vice presidential aide I. Lewis "Scooter" Libby concocted an implausible story in the CIA leak case, while defense attorneys said it would be unfair to convict Libby in a case with so many memory failures.

Libby, the former chief of staff to Vice President Dick Cheney, is charged with lying and obstructing the investigation into the 2003 leak of CIA operative Valerie Plame's identity. Libby told authorities that he learned about Plame from Cheney, forgot about it, then learned it again a month later from NBC reporter Tim Russert.

In closing arguments of the monthlong trial, prosecutor Peter Zeidenberg said it's hard to believe that Libby would forget about Plame since he was eagerly trying to discredit her husband, former Ambassador Joseph Wilson, who had accused the Bush administration of doctoring prewar intelligence on Iraq.

"It's simply not credible to believe he would forget this information about Wilson's wife," Zeidenberg said. "It's ludicrous."

On a TV screen, Zeidenberg displayed a flow chart showing the faces of several Bush administration officials who testified to telling Libby about Plame. From Libby, Zeidenberg drew arrows to people who said Libby talked to them about Plame.

"Is it conceivable that all these witnesses would make the same mistake, the same error in their memory?" Zeidenberg asked.

Defense attorney Theodore Wells portrayed the case as one of competing recollections. Russert says the conversation about Plame never occurred. Wells said it did happen, noted several inconsistencies in Russert's statements and told jurors that the men may simply have different recollections about the same conversation.

"You cannot convict Mr. Libby solely on the word of this man," Wells said. "It would just be fundamentally unfair."

Prosecutors believe Libby feared being fired and prosecuted for discussing official government information about Plame with reporters.

"He had to come up with a story that was innocuous," Zeidenberg said.

So, Zeidenberg said, Libby concocted a story about learning it from Russert rather than Cheney. Russert says that conversation never happened. For that to be believed, Zeidenberg said, Libby had to forget nine conversations about Plame and invent two others.

"That's not a matter of misremembering or forgetting," Zeidenberg said. "It's lying."

Wells told jurors that the prosecution's theory made no sense. Why, he asked, would Libby pin his entire story on Russert? Libby could easily have told investigators he learned about Plame from others who knew about her, Wells said.

"There's no reason, if Mr. Libby was a crook, when he'd make up a lie, he'd ever pick Tim Russert," Wells said.

Zeidenberg also rejected the idea that Libby was made a scapegoat by the White House to protect Karl Rove, Bush's top political strategist. It was just one aspect of a broad defense strategy, but Zeidenberg seized on it almost at the onset of his closing.

"Did you hear any evidence about a conspiracy, a White House conspiracy to scapegoat Mr. Libby?" Zeidenberg asked. "If you think back and draw blank, I suggest to you ladies and gentlemen, it's not a problem with your memory. It's because there was no such evidence."

Wells touched on the idea that Libby was being made a scapegoat for Rove. Wells has described how White House spokesman Scott McClellan publicly declared Rove's innocence in the leak case but failed to do so for Libby. Angered, Libby appealed to Cheney, who intervened and had McClellan exonerate Libby, too.

Fitzgerald says Libby felt guilty about discussing Plame with reporters and was trying to cover his tracks. The Rove theory allows defense attorneys to say that Libby felt wronged by the White House and was acting as an innocent man trying to clear his name.