I don't know what you call "tangible evidence'. Ten handcuffed dead bodies with bullet holes in their heads seem pretty tangible to me. (Photographs of the victims' bodies were included in the McClatchy story.)
And if you were an Iraqi who saw those pictures, how much would YOU trust the US to fairly try the perpetrators?
[Don't get me wrong here: I believe 10 people were murdered in the manner described but I have no way of knowing--and neither do you--whether American troops were responsible or if, instead, the victims were killed by one of the many warring factions in Iraq. The important political fact is that the Iraqis believe American troops were responsible.]
But that's all beside the point of my post which is not to reach any conclusion about the truth or falsity of the atrocity story but rather to explain why we are withdrawing our troops from Iraq. We're getting the usual administration spin about how this "fulfills" Obama's campaign promise and is triumph of American policy. In fact, BUSH signed the Status of Forces Agreement which had a troop withdrawal date of December 31, 2011. Obama tried without success to revise that agreement and extend that date, so for him now to claim that this is fulfillment of a campaign jpromise is BS.