Inside the Raqqa jail where ISIS held their betrayers

(CNN) - It is a place haunted not so much by the ghosts of those imprisoned within, but by the words they left behind on the walls.

Deep inside the dusty bowels of Raqqa's stadium -- an infamous ISIS headquarters from where the terror group is said to have plotted attacks on the Western world -- are a series of makeshift cells.

Some, squat toilets, have been converted into solitary confinement or torture chambers. Yet one room stands out, with names and time served etched onto the white walls, where ISIS held those they suspected of betrayal or espionage.

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