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NEW YORK CBS News appointed former U.S. Attorney General Dick Thornburgh and former Associated Press chief executive Louis Boccardi to investigate what went wrong with its story on President Bush's service in the National Guard.

Thornburgh is a former two-term governor of Pennsylvania and served as attorney general in the administrations of Ronald Reagan and George H.W. Bush.

Boccardi retired last year as president and chief executive officer for The Associated Press. He served on the panel that probed operations at The New York Times following the Jayson Blair scandal.

CBS News admitted on Monday that it could not authenticate documents it had used in a "60 Minutes" story that questioned Bush's service in the Guard during the Vietnam War era. Many critics consider the documents to be fake.

The network and its chief anchor, Dan Rather, have apologized for airing the story.

The two-person panel will begin work immediately, with no deadline set for their investigation, CBS News spokeswoman Sandra Genelius said.

"CBS News has a long tradition of responsible journalism," said CBS co-president Leslie Moonves. "In this case, it's clear that something went seriously wrong with the process. There is no question that the independent review we announced ... is both necessary and important, and I look forward to its thoughts and recommendations.

CBS promised the pair would have full access to everyone at the network and their report would be made public.

The CBS report cited documents purported to be from one of Bush's commanders in the Texas Air National Guard. The documents say the commander, Lt. Col. Jerry Killian, ordered Bush to take a medical exam, which he did not, and felt pressured to sugarcoat an evaluation of then-1st Lt. Bush.

CBS's source, retired Texas National Guard member Bill Burkett, told USA Today that he agreed to turn over the documents to CBS if the network would help arrange a conversation with John Kerry's campaign.

Mary Mapes, a veteran producer at CBS News, passed Burkett's number to the Kerry campaign and Kerry adviser Joe Lockhart said he had spoken to Burkett briefly but did not recall speaking about the National Guard.