By Jane Sutton

GUANTANAMO BAY U.S. NAVAL BASE, Cuba, Feb 6 (Reuters) - A U.S. soldier's account casting doubts on military prosecutors' claims against a young Canadian al Qaeda fighter highlights one inherent difficulty of the Bush administration's efforts to win convictions before war crimes tribunals in Guantanamo.

How to figure out what happened on the battlefield in places like Afghanistan six years later and half a world away?

The account was contained in a five-page document given to reporters this week at a hearing for Canadian captive Omar Khadr, who faces life in prison if convicted of murdering a U.S. soldier with a grenade in Afghanistan in July 2002.

The account was given 20 months later by the American soldier who shot Khadr twice in the back during the battle and says that one other man inside that bombed-out compound was still alive when U.S. forces entered.

That left open the possibility that someone other than Khadr could have thrown the grenade, contradicting accounts by another participant who had said in media interviews that it had to have been Khadr because he was the only one left alive.

The chief military prosecutor, Army Col. Larry Morris, said it was not surprising the accounts would differ when bullets and grenades were flying.

"That is a window into what happens at war -- lots of people with fleeting perspectives in short periods of time, trying to remember their best, trying to perceive as best, as quickly as they can and then report it as accurately as they can," Morris said on Wednesday.

Journalists were inadvertently given an unredacted version of the document, then asked to return it, then allowed to keep it but asked not to use the names, unit numbers and other identifying data it contained.

SECRET INFORMATION

As a condition for visiting the remote U.S. naval base in Cuba to report on the widely criticized trials, journalists must sign an agreement not to divulge secret information and acknowledge that they could be prosecuted if they do.

But Morris said the document was never "secret." He said Khadr's defense lawyers have had it for years and prosecutors take seriously their duty "to provide every scrap of evidence to the defense that is potentially exculpatory or might mitigate or reduce his guilt."

He also said the public would be allowed to see all the evidence given to the jurors at trial.

The chief defense attorney, Army Col. Steve David, said he has no reason to believe prosecutors are withholding evidence. But military defense lawyers have been struggling to obtain evidence that is in the hands of numerous government agencies, some with reason to keep their files secret.

"It underscores another issue and that is the issue with classification and who classifies, who unclassifies or reclassifies," David said.

"There are layers and multiple entities and agencies involved. It's a function of asking the right questions ... But if I don't have it, I can't give it to you if I don't know where to ask."

Military defense lawyers have been denied permission to interview "high-value" prisoners transferred to Guantanamo after being held in secret CIA custody. They believe those allegedly high-ranking al Qaeda operatives have information that could help them understand what, if any, role their clients had in the organization.

"It raises the issue of what the facts are, what actually happened," David said.

Defense lawyers also suspect some evidence was not preserved because no one ever expected to need it.

"Did that soldier on the battlefield anticipate that they were going to be involved in a criminal prosecution?," David asked. "I don't think we anticipate a criminal prosecution of a 15-year-old."

Khadr, now 21, was 15 when captured and 16 when sent to Guantanamo. (Editing by Michael Christie)

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