The mass grave contained the victims who had been shot or beheaded by the ultra-radical militants, said residents nearby. They also said they believed up to 200 people were killed in the weeks before Islamic State withdrew from the town.
Human Rights Watch said some of the former policemen were separated from a group of about 2,000 people from nearby villages and towns who were forced to march alongside the militants last month as they retreated north to Mosul and the town of Tal Afar.
It quoted a labourer who said he saw Islamic State fighters drive four large trucks carrying 100 to 125 men, some of whom he recognised as former policemen, past an agricultural college close to the site which was to become the mass grave.
Minutes later, he heard automatic gunfire and cries of distress, he said. The next night, on October 29, a similar scene was repeated, with between 130 to 145 men, he told HRW.
Another witness, a resident of Hammam al-Alil, said he heard automatic gunfire in the area for approximately seven minutes, three nights in a row.