Islamic State built prison cells inside an ordinary villa in Mosul as part of the group's tactic of boosting the safety of its jails and security centers by concealing them among regular houses.
Despite their outward appearance, the jails had the regular attributes of state-run detention centers where inmates have their documents and belongings taken away when they arrive, to be given back to them if and when they are released.
He said former members of the Iraqi armed forces were kept in this cell, along with Yazidis, the religious community who suffered the most under Islamic State because the hardline group considered them devil worshippers.
"Here they kept 'violators'" of their laws, he said. "But they are the real violators" in the eyes of Iraqi law, he added.