The US is accused of mistreating detainees
Two Afghan prisoners were killed while in US custody at their base at Bagram, a military coroner has concluded.
The report said "blunt force trauma" had contributed to the deaths.
The detainees had spent about a week in the detention facility when they died last December.
However, US spokesman Colonel Roger King told BBC News Online the pathologists' verdict was not final - a military investigation had been launched and was due to be completed later this month.
There are hundreds of former Taleban and al-Qaeda prisoners held at Guantanamo Bay in Cuba and in various overseas facilities.
Last month, human rights groups accused the US Government of subjecting the prisoners to physical abuse leading to a number of deaths and attempted suicides in custody.
Washington described the allegations of torture as "ridiculous".
The US spokesman at Bagram said the two men who died there had been under allied custody for about 10 days altogether.
The first man died on 3 December after a blood clot in his lungs, and the second died a week later after developing blood clots as well as suffering a heart attack.
But Colonel King vehemently denied the prisoners had been mistreated by US forces.
The homicide entry on the [military death certificate] form is different from the legal meaning of the term
Colonel Roger King
US spokesman at Bagram
"They are the first detainees to have required medical treatment at the Bagram facility," he said, and "the only casualties" so far.
Pathologists, he said, had a limited choice when filling the military death certificate.
Specific allegations of prisoner torture were first published in the Washington Post in December last year.
According to the paper, interrogators from the Central Intelligence Agency (CIA) had been subjecting Taleban and al-Qaeda suspects to "stress and duress" techniques of dubious legality.
Suspects at US facilities in Afghanistan and other foreign countries were sometimes held in uncomfortable positions for hours and deprived of sleep, the paper alleged.
About 650 men have been at Guantanamo Bay since the detention base was established in January 2002. Many more are held elsewhere.