As many as 13,000 opponents of Syrian President Bashar Al-Assad were secretly hanged in one of the country’s most infamous prisons in the first five years of the country’s civil war, Amnesty International said Monday, according to The Guardian.

Many thousands more people held in the Saydnaya prison died through torture and starvation, the group said in its latest report which details allegations of state-sanctioned abuse that are unprecedented in Syria’s civil war.