As Iraqi-led forces liberated the town of Hamam al-Alil south of Mosul in Nineveh, federal police uncovered at least 100 decapitated bodies near a university that was apparently used by ISIS as a mass grave.

Most of the victims have not been identified, as they have already decayed into skeletons. Authorities also could not yet tell if the victims were civilians or security forces.  

Federal police going around the liberated town on Monday noticed a strong smell and found the headless bodies near the Faculty of Agriculture.

"Iraqi forces, having liberated Hamam al-Alil neighborhood, and inside the Faculty of Agriculture, ran into a new crime discovering one hundred dead bodies of citizens," Iraqi officials said, according to Iraqi News. "Specialized teams will be sent to investigate that heinous crime."

The site appeared to be a parcel of wasteland beside the school, according to BBC. Members of the media were not afforded a closer look of the mass grave because there were still improvised explosive devices planted on the road going to the Faculty of Agriculture.

According to witnesses, the Islamic State has conducted mass killings in recent weeks as Iraqi forces continued to advance toward Mosul.

A former English teacher in the town, named Riyad Ahmed, said that he often heard the cries of the torture victims in a makeshift jail near his house. Sometimes he would peer out of his window and see ISIS fighters dragging their victims into the jail.

He said the terror group has executed many people in his town. On the street where he lives, ISIS killed six people, including his father and a family that lived next door.

"The devil himself would be astounded by Daesh's methods of torture. It is beyond the imagination," Ahmed told Reuters, referring to ISIS using its Arabic acronym. "They would torture them inside and then take them out of the neighborhood and either shoot them or slit their throats."

They even put a bullet in the head of a one-year-old baby, he said.

Ahmed said he and his family joined about a hundred other town residents in one house to hide from ISIS fighters, making sure their presence would not be detected.

"They didn't know we were here. We didn't make a sound. No lights, no sound, no speaking at all," Ahmed said. If ISIS had spotted them, they would have been taken to Mosul where the Islamic State fighters have fled, he said.

Other reports claimed that ISIS killed dozens of residents in the town in a period of just one week after they suspected some people would mount a rebellion against them in support of the Iraqi troops pushing forward to Mosul.

U.N. spokesperson Ravina Shamdasani said ISIS fighters abducted 295 Iraqi Security Forces just in the past week. They also could have brought 1,500 families with them as they retreated to Mosul to use them as human shield.