The fall of the Bashar al-Assad regime did not free all the prisoners in Syria.
Al Sina prison holds more than 5,000 prisoners in the Kurdish-controlled northeast of the country.
These prisoners – including some Britons – are suspected fighters and supporters of the so-called Islamic State group. Most are said to have been loyal to him until his last position in 2019.
The BBC's senior international correspondent, Orla Guerin, was granted rare access to visit the prison.