In December 2001, Afghanistan’s Northern Alliance, with strong U.S. backing consisting of special-forces units and CIA paramilitary operatives, were close to consolidating their control over the country. Kabul was occupied, and Kunduz, the last major Taliban stronghold in the north, had been crushed. Large numbers of Taliban forces and their allies had surrendered.

Then, in the north, as many as 2,000 prisoners who had surrendered to the Alliance or their American supporters were apparently shot to death or suffocated in sealed metal truck containers while being transferred to Afghanistan’s Sheberghan prison. The dead prisoners from this “convoy of death” were then buried in the northern Afghanistan desert, at Dasht-e-Leili. By the next year, many of the bodies had been exhumed and examined. Some of them bore clear signs of torture.

The incident is without doubt the most serious war crime arising out of the U.S. and Northern Alliance campaign to defeat the Taliban and establish a new regime in Afghanistan. To the best of our knowledge, Americans do not appear to have been involved in carrying out the atrocities, which were reportedly carried out by forces controlled by General Rashid Dostum, an ethnic Uzbek warlord who commanded forces in the vicinity of Mazar-e-Sharif, on the Uzbekistan frontier. The Rumsfeld Pentagon disclaimed the U.S. responsibility to investigate the incident on these grounds, and strained to cover up the incident. But it was later established that a significant number of American advisers were on hand at the time of the massacre.

Following these disclosures, in July 2009, CNN’s Anderson Cooper pressed President Barack Obama about the incident. Obama stated that he would ask his national security team “to collect the facts” and would “make a decision on how to approach it once the facts were known.” More than two years have passed since this pledge, but no further evidence has emerged, and no statement or report has been produced to show that an investigation was conducted.

To mark the tenth anniversary of the Dasht-e-Leili massacre, Physicians for Human Rights (PHR), an organization that played a key role in uncovering the scope of the incident, has written President Obama to remind him of his promise. “PHR urges you to review and make public the results of your investigation into the Dasht-e-Leili massacre and to address accountability by the U.S. and Afghanistan in regard to this atrocity,” the organization writes.

Nations rarely want to fully expose their involvement in mass human-rights violations. That is true even of countries with an otherwise respectable record of observing the Geneva Conventions and other international instruments of justice. In the case of the Dasht-e-Leili massacre, it is fairly easy to envision how figures within the Pentagon and CIA would push back against an effort to expose the facts. Echoing the Rumsfeld era, they would argue that the massacre was carried out by Dostum’s people, and that they had nothing to do with it. American personnel were present only in an advisory role. Moreover, they might add, uncovering the truth about Dasht-e-Leili would only complicate an already difficult political balancing act for Kabul — Dostum is now viewed as the leader of the nation’s Uzbeks, after all, and reconciling him to Kabul is essential if the Afghan government wishes to hold the north against the resurgent Taliban.

This line of argument could be true, or it could be obscuring darker truths. Exposing atrocities is always politically messy. Nevertheless, an honest, thorough investigation and exposure of the facts, no matter how unpalatable they may be, is a legal and moral obligation for the United States. During the Bush years, the Pentagon discharged that responsibility impressively when ordinary soldiers were involved — but the minute senior political figures or their policy decisions were implicated, a snow storm of obfuscation and denial brought inquiry to a standstill. There is every reason to suspect that the same pattern exists today.

And so, President Obama’s promise to Anderson Cooper seems to have faded in favor of political expedience. Pledging to “look forward, not back” will not erase the stain of Dasht-e-Leili. It will only associate that stain more strongly with the culture of unaccountability in Washington.