The massacres ended when the Vietnamese military invaded in 1978 and toppled the Khmer Rouge regime. By January 1979, 1.5 to 2 million people had died due to the Khmer Rouge's policies, including 200,000–300,000 Chinese Cambodians, 90,000 Muslims, and 20,000 Vietnamese Cambodians. 20,000 people passed through the Security Prison 21, one of the 196 prisons the Khmer Rouge operated, and only seven adults survived. The prisoners were taken to the Killing Fields, where they were executed (often with pickaxes, to save bullets) and buried in mass graves. Abduction and indoctrination of children was widespread, and many were persuaded or forced to commit atrocities. As of 2009, the Documentation Center of Cambodia has mapped 23,745 mass graves containing approximately 1.3 million suspected victims of execution. Direct execution is believed to account for up to 60% of the genocide's death toll, with other victims succumbing to starvation, exhaustion, or disease.