As the Assad regime in Syria crumbled over the weekend, the conquering rebels threw open the gates to the prisons where the government had detained tens of thousands of its people, torturing and killing them on an industrial scale.

Throngs of Syrians rushed to the facilities to search for loved ones who had disappeared into the prison system during the 13-year civil war. No prison is more infamous than Sednaya, just north of the capital, Damascus.

Even before the civil war, Sednaya was known for widespread torture and abuse. But during the conflict, it became a site of depravity and violence, used to commit some of the worst atrocities of Bashar al-Assad’s rule when he was president.

Human rights groups say tens of thousands of people were detained in Sednaya. They were tortured, beaten and deprived of food, water, medicine and basic sanitation. Thousands were executed in mass hangings after sham trials. One group estimated that more than 30,000 detainees were killed there.

In most cases, the families of prisoners were given no information about their fate.