The Khmer Rouge regime arrested and eventually executed almost everyone suspected of connections with the former government or with foreign governments, as well as professionals and intellectuals.
According to Michael Vickery, 750,000 people in Cambodia in a population of about 8 million died due to disease, overwork, and political repression. However many scholars disregard his claims because the number of victims of execution found in the mass graves is higher than his estimate for deaths from all causes during the rule of the Khmer Rouge and the civil war combined.[7]
The most widely accepted estimate, from scholar Ben Kiernan, is that about 1.7 million people were killed. It is described by the Yale University Cambodian Genocide Program as, "one of the worst human tragedies of the last century."[8] Researcher Craig Etcheson of the Documentation Center of Cambodia suggests that the death toll was between 2 and 2.5 million, with a "most likely" figure of 2.2 million.
After 5 years of researching some 20,000 grave sites, he concludes that, "these mass graves contain the remains of 1,386,734 victims of execution."[7]
Execution is believed to have accounted for about 30–50% of the death toll. This would indicate 2.5 to 3 million deaths, but normal mortality over this period would have accounted for about 500,000 deaths—subtracting this from the total sum, we arrive at Etcheson's range for the number of "excess" deaths attributable to the Khmer Rouge regime