Islamabad, Pakistan ? A confidential U.N. memorandum found evidence to justify a “full-fledged criminal investigation” into the deaths in Afghanistan of hundreds of Taliban prisoners held by the U.S.-backed northern alliance, Newsweek reported Sunday.

Citing the memo, Newsweek said U.N. investigators based their finding on an investigation of a mass grave that “contains bodies of Taliban POWs who died of suffocation” while being transferred from Kunduz to a prison at Shibergan after Taliban resistance in northern Afghanistan collapsed in November.

The magazine said the memo referred to “political sensitivity” and recommended a halt to “all activities relevant to this case” until a decision was made on whether to push for a criminal trial, truth commission or other alternatives.

Northern Afghanistan had one of the worst records of mass killings on all sides during years of fighting that began when the Soviet Union invaded in 1979. The United Nations investigated a report that as many as 2,000 Taliban were massacred in 1997 around Mazar-e-Sharif. A year later, the Taliban were accused of slaughtering thousands of ethnic Hazaras in revenge when the hardline Islamic militia recaptured the city.

Last November, thousands of Taliban and al-Qaida fighters, including American John Walker Lindh, surrendered to northern alliance forces in the Kunduz area following intensive U.S. air attacks.

Many of the prisoners were transferred in metal containers to Shibergan, the stronghold of infamous Uzbek warlord Abdul Rashid Dostum, who was allied with the United States in the war against the Taliban and is now President Hamid Karzai’s special representative in the north.

In Boston, the group Physicians for Human Rights, which also sent a team to investigate the reported massacre, said it had repeatedly asked the governments of the United States and Afghanistan as well as the United Nations to secure the gravesite and launch a comprehensive criminal investigation.

“The refusal of the United States to acknowledge and investigate the possibility that its military partner murdered hundreds or thousands of prisoners is a terrible repudiation of its commitment to hold perpetrators of war crimes accountable for their deeds,” Leonard Rubenstein, executive director of Physicians for Human Rights, said in a statement released Sunday.

Following repeated reports of mass deaths among prisoners, U.N. and human rights investigators visited the area in the spring. In May, the United Nations told reporters in Kabul that a U.N. team of forensic scientists found evidence of widespread death by suffocation among bodies uncovered in a large mass grave Shibergan.

The United Nations said at the time that the grave was believed to hold the bodies of Taliban prisoners who died in the hands of their northern alliance captors. However, no recommendation was made publicly on how to pursue the case.

An undetermined number of U.S. special operations forces were in the north at the time of the Kunduz surrender. In its report, Newsweek said “extensive inquiries” found nothing to suggest that U.S. troops “had advance knowledge of the killings, witnessed the prisoners being stuffed into the unventilated trucks, or were in a position to prevent that.”