A mass grave containing the remains of about 500 people has been found at Badoush prison near Mosul, Iraqi forces say.
The Shia-led Hashd al-Shaabi forces said the victims were "civilian prisoners" murdered by militants from so-called Islamic State (IS).
IS is alleged to have killed up to 600 inmates, most of them Shia Muslims, when it seized Mosul in 2014.
Badoush prison was recaptured earlier this week.
The Hashd al-Shaabi paramilitary forces said on Saturday that the "large mass grave" contained the remains of about 500 civilian prisoners who were "executed by (IS) gangs after they controlled the prison during their occupation of Mosul".
However, an army source deployed with Hashd forces was unable to confirm the discovery.
Although Saturday's report could not be immediately verified, a 2014 report by Human Rights Watch said that hundreds of male inmates were murdered by IS gunmen at the time.
After Badoush prison was seized in June 2014, some 1500 inmates were rounded up and transported by lorry to an isolated stretch of desert about 2km away, survivors told the rights group.
According to the group's report, IS gunmen separated the Shia inmates from the Sunnis and Christians and then marched them to a ravine, where they were forced to kneel along its edge.
The Shia inmates were shot in the head or back with assault rifles and automatic weapons, according to the survivors, who escaped by pretending to be dead or because they were shielded by the bodies of victims who fell on top of them.
Last year, a mass grave containing the remains of about 100 beheaded civilians was discovered inside the grounds of a school in Mosul.
The Iraqi army's 9th Armoured Division and allied militiamen recaptured Badoush prison, north-west of Mosul, on Wednesday.
A statement released by the Iraqi forces did not say whether they found anyone who was being detained by IS.
Iraq's government launched an operation to retake Mosul in October, and announced that the city's eastern side had been liberated in January.
Troops are now pushing into the more densely-populated west.