At Saydnaya Military Prison, the Syrian authorities have quietly and methodically organized the killing of thousands of people in their custody. Amnesty International’s research shows that the murder, torture, enforced disappearance and extermination, carried out at Saydnaya since 2011, has been perpetrated as part of an attack against the civilian population that has been widespread, as well as systematic, and carried out in furtherance of state policy. We therefore conclude that the Syrian authorities’ violations at Saydnaya amount to crimes against humanity. Amnesty International urgently calls for an independent and impartial investigation into crimes committed at Saydnaya.