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WASHINGTON (CBS News) ― Jurors began deliberating Wednesday in the perjury and obstruction trial of former White House aide I. Lewis "Scooter" Libby.

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.

Jurors heard about an hour of legal instructions from U.S. District Judge Reggie B. Walton on Wednesday morning before beginning deliberations shortly before 11:30 a.m. They heard a full day of closing arguments Tuesday after a monthlong trial.

The jury of eight women and four men must be unanimous before returning a verdict on the five charges against Libby. He faces up to 30 years in prison if convicted, though he'd likely get far less under federal sentencing guidelines.

Plame is married to former Ambassador Joseph Wilson, who emerged in mid-2003 as an outspoken critic of the Bush administration's case for the Iraq war. Special Prosecutor Patrick Fitzgerald says Libby learned about Plame from Cheney and other officials in June 2003 and relayed it to reporters.

Attorneys left jurors with two very different impressions of the case. Prosecutors described a methodical effort by Libby to conceal the fact that he discussed Plame with reporters in June 2003. Defense attorneys recounted a confusing, sometimes contradictory month of testimony that they believe is too shaky to base a conviction on.

CBS News legal analyst Andrew Cohen says that if Libby is to be acquitted, jurors "will have to give him the benefit of the doubt. They're going to have to believe that this was an innocent mistake and that he's too smart to have gotten caught in this perjury trap that was laid for him."

The problem with that argument, Cohen says, "is if you believe it you have to disbelieve Bob Woodward and Tim Russert and Walter Pincus and all these people who really don't have a whole lot of motive to lie to get Libby in trouble. That's the strength of the prosecution case and the weakness of the defense case."

Plame was outed by reporter Robert Novak, who touched off an FBI investigation with a July 2003 syndicated column. Though Libby wasn't Novak's source, prosecutors say he feared he'd be charged with discussing classified information with other reporters.

So, prosecutors say, Libby lied. He told investigators he learned about Plame from Cheney, then forgot about it until a month later, when he was surprised to hear it during a phone call with NBC's Tim Russert. Russert says the conversation about Plame didn't happen.

"It's simply not credible to believe he would forget this information about Wilson's wife," prosecutor Peter Zeidenberg said during closing arguments Tuesday. "It's ludicrous."

Libby's attorneys say it's the prosecution's story that doesn't make sense. Why, they ask, would Libby make up a story that hinges on Russert, someone Libby knew could be questioned?

And while prosecutors have shown that Plame came up in several conversations with government officials in mid-2003, defense attorneys say there's no proof Libby deliberately set out to lie when he spoke to investigators three months later.

"They haven't given you anything that says Mr. Libby didn't just have one of those moments that we all have in life where he thought something happened one way and it happened another," said defense attorney Theodore Wells.

Prosecutors used opening statements to remind jurors that one witness after another testified to discussing Plame with Libby. Zeidenberg pointed to a flow chart showing arrows tracking information from several officials to Libby and on to other sources. With each conversation, it became less likely the CIA operative would just slip Libby's mind, Zeidenberg said.

Wells presented a similar chart. Under each name and photograph, however, Wells noted their memory inconsistencies. Some changed their stories. Others seemed less certain in FBI interviews than they did on the stand.

Why is Mr. Libby being treated differently?" Wells told jurors. "He should not. Treat him fairly. Ask yourself: 'Did the government give me some evidence that he intentionally lied as opposed to, did he just forget?"'

(© 2007 CBS Broadcasting Inc. All Rights Reserved. This material may not be published, broadcast, rewritten, or redistributed. The Associated Press contributed to this report.)