(CBS) Mary Tillman believes she has evidence that the Army went to extraordinary lengths to keep what really happened under wraps. She points to the fact that Pat’s uniform, which according to one soldier had the marks left by American bullets, was burned, which is against Army procedure.

Then there was the coroner, who'd refused to sign the autopsy for months because when he examined the body, he said the gunshot wounds were not consistent with the Army's original story. And remember the citation on the Silver Star which left the distinct impression Pat was killed by enemy fire? It was later revealed that the eyewitness statements had been altered. Army Ranger Bryan O'Neal testified before Congress last spring.

"Did you write these sentences, claiming that you were engaged with the enemy?" he was asked.

"No sir," O'Neal replied.

"Their testimony was altered by someone to make it appear as though Pat was killed by the enemy," Mary Tillman claims.

But she says she hasn't figured out who changed the statements or how.

Couric asked Pete Geren, the new secretary of the Army, how this could have happened.

"Who do you think, Secretary Geren, altered those soldiers' statements when it came to that Silver Star?" Couric asks.

"Well, that's one of the questions that we will never completely answer," Sec. Geren says. "But it certainly is one of the areas that that raises questions. There are so many mistakes. So many things that happened. If you add them all together, it certainly calls into question the credibility of those who handled this. And raises the kind of questions that Ms. Tillman raises. I don't blame her for that. And I don't expect her ever to believe us. But there was no effort to deceive. There were mistakes and grievous errors by the legions. And as a result, we fell short of our duty to her as a mother of one of our heroes."

"If there was no effort to deceive, why wasn't she told right away that her son's death was being investigated as a possible friendly fire incident?" Couric asks.

"She should have been and Army policy requires that she be notified right away. This was a classified mission, and the people in the operation mistakenly believed that they were not supposed to release any of the details of the investigation until the investigation was complete. They released it 35 days later. But, we've had seven investigations and they have all concluded that there was no deceit, no intentional deceit, no cover up," Geren says.

Produced by Kyra Darnton
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