Chicago ? CBS News announced Wednesday that a Republican politician and a retired news executive will lead the independent investigation into how the network came to air a story this month that relied on what many experts believe are fake documents.

Dick Thornburgh, former governor of Pennsylvania and attorney general under Presidents Ronald Reagan and George H.W. Bush, and Louis Boccardi, who retired last year as president and chief executive of The Associated Press, will begin their work this week, the network said in a statement. They “will have full access and complete cooperation from CBS News and CBS, as well as all the resources necessary to complete the task,” the network said.

Questions about how CBS News producer Mary Mapes obtained the documents, which CBS admitted Monday it could no longer vouch for, have become a campaign issue, with the Bush camp accusing the Kerry campaign of orchestrating a smear.

Observers said CBS likely chose a Republican for the investigation to insulate itself from charges that it was coordinating with Democrats.

CBS said Mapes got the documents from Bill Burkett, a former member of the Texas National Guard and longtime Bush detractor who said he got them from a woman named Lucy Ramirez. But the Bush campaign has pounced on revelations that Mapes put Burkett in touch with Kerry aide Joe Lockhart, suggesting that the Kerry campaign was somehow behind the story.

“The investigation is important,” said Claire Buchan, a White House spokeswoman. “But it’s equally important that the Kerry campaign come forward about their involvement in coordinating these attacks on the president.”

A spokesman for Kerry dismissed that accusation as “absurd.”

Asked in Florida about the issue, Kerry said: “I can’t speak to the CBS thing.”

Boccardi, a 36-year veteran at the AP, said he had no reason to doubt Thornburgh’s ability to remain nonpartisan in the investigation.

Boccardi said the investigation, which will be assisted by staff from Thornburgh’s law firm, Kirkpatrick & Lockhart, would focus on “the journalism that went on — how it got to air, and what was done that shouldn’t have been done and what wasn’t done that should have been done.”

He also said he intended to investigate why CBS executives strongly defended the story initially in the face of increasing scrutiny.

Thornburgh declined to comment, as did CBS News president Andrew Heyward.

Inside CBS News, weary staffers said the investigation wouldn’t do much to restore the network’s credibility.

“What more do we need to know?” said one CBS News staffer. “We know that CBS News can reasonably be suspected of abetting fraud in the hopes of influencing the outcome of an election. We know that (the producers of the story) ran roughshod over a battery of journalism standards and vetting procedures.”