Omar Khadr's lawyers face almost insurmountable hurdles in defending the Canadian against war crimes, because the military-commission process under which he is being tried is stacked against him, experts watching the proceedings in Guantanamo Bay say.

The observations come amid fresh testimony Thursday from an interrogator, who threatened the captive teenager with stories of Afghan prisoners being gang raped in U.S. prisons to get him to talk.

The testimony was part of defence efforts to have self-incriminating statements Khadr made thrown out as the product of torture.

Still, Audrey Macklin, a University of Toronto law professor, said the rules "cripple the defence."

Macklin said the prosecution was largely relying on statements Khadr gave to "clean-teams" — friendly and unthreatening interrogators — while the defence maintains the clean teams were sent in between abusive interrogations.

However, the prosecution has refused to name the interrogators who may have tortured or abused Khadr and the judge, Col. Patrick Parrish, has refused to make them testify.

In addition, Macklin said, the judge has been unwilling to hear "similar-fact evidence" that other detainees at Bagram prison in Afghanistan and at Guantanamo Bay were tortured or abused.

"The practice is to make it impossible for the defence to access and bring forward the evidence that would prove torture," Macklin said from Guantanamo Bay.

"Similar-fact evidence is important here because the only witnesses to Omar's interrogation are Omar and his interrogators, (who) are not likely to admit to doing anything improper."

Now 23, the Toronto-born Khadr is charged with throwing a hand grenade that killed an American special forces soldier after a U.S. military assault on a compound in Afghanistan in July 2002, when he was 15 years old.

On the eighth day of pretrial hearings Thursday, a former U.S. Army interrogator testified that he told the badly wounded teen a "fictitious" tale of an Afghan youth who was gang-raped in an American prison and died.

"We'd tell him about this Afghan (who) gets sent to an American prison and there's a bunch of big black guys and big Nazis," the witness, identified as Interrogator No. 1, told the hearing.

The interrogator was attached to the 519 MP Battalion, which guarded prisoners at Bagram air base in Afghanistan in 2002.

Three years later, he pleaded guilty to three acts of detainee abuse on another captive at Bagram.

Interrogator No. 1 said he grilled Khadr as many as 25 times over 100 hours before the teen was sent to Guantanamo for more interrogations.

The rape story, he said, was part of a technique designed to break particularly unco-operative prisoners.

He also admitted screaming at Khadr and knocking a bench over with a bang.

Khadr has claimed to have been threatened with rape, among other abuses.

For their part, prosecutors deny Khadr was abused.

They called other interrogators to testify that the Canadian told them voluntarily and truthfully that he threw the grenade, and that he planted explosives meant to kill American soldiers and earn him $1,500 a head.

Like Macklin, Tom Parker, with Amnesty International USA, said Parrish appeared to consistently uphold prosecution objections to defence efforts to raise similar-fact evidence.

"The cards are stacked heavily against Omar Khadr," said Parker, who also attended the proceedings.

Parker noted one interrogator described Khadr as a "young man who never had a chance to grow up."

"I have the growing sense that it is unlikely that Omar Khadr is going to be afforded the opportunity to resume that journey any time soon," Parker said.

The hearings have recessed until possibly next month.

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