Two years after the IS onslaught in northern Iraq, the investigators, as well as US diplomats, say the Obama administration has done little to pursue prosecution of the crimes that Secretary of State John Kerry has called genocide.
Current and former State Department officials say that an attempt in late 2014 to have a legal finding of genocide was blocked by the Defense Department, setting back efforts to prosecute IS members suspected of committing war crimes.
Officials in Washington say that the Defense Department and ultimately the administration were concerned that court trials would distract from the military campaign. But the diplomats say that justice is essential in a region whose religious minorities have been terrorized.
The officials spoke on condition of anonymity because they were not authorized to discuss the issue.
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Stephen Rapp, who stepped down as the administration's ambassador at large for war crimes last year, says the administration should have moved early to help secure evidence of IS atrocities and push for the creation of special Iraqi courts to try war crimes.
"The priority for the US government is to win the war against the Islamic State and destroy them," Rapp said.
Rapp is now the chairman of the advisory board of the commission, whose investigators in Iraq work with the Kurdish regional government to formally document the IS group's crimes, including those against the Yazidi minority group.
They have built a case implicating the entire IS command structure in a plot to kidnap Yazidi women and girls and establish a sex-slave market.