Ben Kiernan estimates that about 1.7 million people were killed. 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". A UN investigation reported 2–3 million dead, while UNICEF estimated 3 million had been killed. Demographic analysis by Patrick Heuveline suggests that between 1.17 and 3.42 million Cambodians were killed, while Marek Sliwinski suggests that 1.8 million is a conservative figure. Even the Khmer Rouge acknowledged that 2 million had been killed—though they attributed those deaths to a subsequent Vietnamese invasion. By late 1979, UN and Red Cross officials were warning that another 2.25 million Cambodians faced death by starvation due to "the near destruction of Cambodian society under the regime of ousted Prime Minister Pol Pot", who were saved by international aid after the Vietnamese invasion.