Zakrya Kandahari, who worked as an interpreter, was arrested by Afghanistan's premier intelligence agency around six weeks ago.
According to a copy of a report available with AFP and confirmed as authentic by a security official, intelligence agents have a video showing Kandahari beating a prisoner.
Afghan authorities are investigating allegations that armed Afghans working with US special forces harassed, tortured and murdered civilians in Wardak province, a Taliban flashpoint on the doorstep of Kabul.
"Zakrya himself has denied the accusations, saying he was under the command of others," said a copy of the report obtained by AFP.
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Instead he blamed the murders on three Americans, whom he named as Dave, Hagen and Chris, and whom he said were fluent in Afghanistan's two main languages Dari and Pashto.
"I was simply a low-rank translator and did not have authorisation to roam around inside the base, or (go) to interrogation sections," the report quoted Kandahari as saying.
He was picked up in the southern city of Kandahar, the spiritual capital of the Taliban, and moved to Kabul for interrogation, officials said.
US investigations have found "no credible evidence" to substantiate any allegations of abuse by either NATO or US forces, a military spokesman told AFP today.
But the military is co-operating with the Afghan government and US Army criminal investigators are also looking into the claims, said Lieutenant Colonel Will Griffin from the US-led NATO force.