Buddhists monks use mobile phones to take pictures of a performance based on the Khmer Rouge regime during the annual “Day of Anger” ceremony at the Choeung Ek “Killing Fields” site, located on the outskirts of Phnom Penh

Buddhists monks use mobile phones to take pictures of a performance based on the Khmer Rouge regime during the annual "Day of Anger" ceremony at the Choeung Ek "Killing Fields" site, located on the outskirts of Phnom Penh, May 20, 2015. Thousands of Cambodians, including monks, gathered at the site to remember those who perished during the radical communist Khmer Rouge's 1975-79 regime. REUTERS/Samrang Pring

Buddhists monks use mobile phones to take pictures of a performance based on the Khmer Rouge regime during the annual “Day of Anger” ceremony at the Choeung Ek “Killing Fields” site, located on the outskirts of Phnom Penh, May 20, 2015. Thousands of Cambodians, including monks, gathered at the site to remember those who perished during the radical communist Khmer Rouge’s 1975-79 regime. REUTERS/Samrang Pring