Iraqi forces, who continue their advance in western Mosul, said that they had found a mass grave containing the remains of about 500 people at Badoush prison on Saturday (11 March).
The Hashd al-Shaabi forces, an umbrella pro-Iraq group comprising Iran-backed Shia militias said that the remains found were of "civilian prisoners" who were "executed by (Isis) gangs after they controlled the prison during their occupation of Mosul".
Paramilitary forces have discovered dozens of mass graves containing hundreds of bodies as they battle Isis militants to retake the city from their control. The extremists had allegedly murdered up to 600 prison inmates, mostly Shia Muslims, when they captured Mosul in 2014.
However, an army source deployed with the Hashd forces could not confirm the discovery, the BBC reported.
A report by Human Rights Watch released in 2014 said that hundreds of male prisoners were killed by Isis militants at the time.
After the militants captured the prison in June 2014, at least 1,500 men were rounded up and transported to an isolated place in the desert, according to survivors.
The HRW report states that they separated Sunni prisoners from Shia and Christians and took them to a nearby ravine and shot them with assault rifles.
The jihadists' atrocities extended beyond prisoner executions. According to an Iraqi lawmaker, Vian Dakhil, around 500 Yazidi women were held at the Badoush prison and were used as sex slaves.