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This implies that about two hundred (200) times as many extra Iraqis have been killed since the U.S. invasion as the number of Americans killed on 9/11. While most of the deaths in the first year were attributed to the U.S.-led forces, most of the deaths since are attributed to insurgents. The study controlled for reporter bias by checking actual death certificates and finding them in 92% of the cases -- pretty good record-keeping and truth-telling for such a chaotic land.
Here is the study. Here is a good discussion of the study.