A mass grave containing the remains of about 100 beheaded civilians has been discovered inside a school in a town south of Mosul.
The mass grave was found by Iraqi forces in the School of Agriculture on the outskirts of Hammam al-Alil, a town recaptured from Islamic State yesterday.
The grisly discovery came as Iraqi forces faced fierce opposition from Islamic State fighters to their push further into the city.
Special teams were being sent to the site to investigate the killings.
Beheadings and mass graves have become a hallmark of the terror group.
Hammam al-Alil resident Riyad Ahmed, 29, told Reuters he would peer out from his window at Islamic State fighters dragging civilians into a makeshift jail across the street and then sending them in the middle of the night to be executed.
The former English teacher recalled hearing victims' cries of agony as he hid with dozens of neighbours in the shadow of one of the group's detention centres.
"The devil himself would be astounded by Daesh's methods of torture. It is beyond the imagination," Mr Ahmed said, using an Arabic acronym for Islamic State.
Iraq's army and federal police, participating in a US-backed offensive launched last month to recapture the largest population centre under the jihadists' control, retook the area over the weekend.
As the forces advance, details of Islamic State's brutality and growing desperation, which have trickled out of its self-proclaimed caliphate over the past two years, are being reinforced by first-hand accounts of residents.
Mr Ahmed said no part of Hammam al-Alil had been spared from the ultra-hardline Sunni Islamists' violence.
In his street alone, six people he knew had been executed, including his father and a family of three that lived next door, he said.
Aid organisations, local officials and Mosul residents have said that Islamic State executed dozens of people in Hammam al-Alil and barracks nearby over the course of a week, on suspicion of planning rebellions in and around Mosul to aid the advancing troops.
Nineveh provincial council member Abdul Rahman al-Waggaa told Reuters last month that most of the victims were former police and army members.
Islamic State had used the town's agricultural college as "a killing field" for hundreds of people in the days before the Iraqi government advance, Mr Ahmed said.
"They would torture them inside and then take them out of the neighbourhood and either shoot them or slit their throats."
Police backed up his accounts, but the road to the college was still lined with improvised explosive devices, preventing Reuters from visiting.
'We have been living in hell'
Ahmed, who learned English when US forces occupied Iraq for nine years after toppling Saddam Hussein in 2003, was delighted to speak to a foreign reporter after two years during which he feared he would be killed for using English.
"We have been living in hell, like zombies," he said.
Residents still in Hammam al-Alil told how they packed into homes with nearly 100 other people each for days to avoid being forced to flee to Mosul as Islamic State retreated.
"They didn't know we were here. We didn't make a sound. No lights, no sound, no speaking at all," Mr Ahmed said.
His family had stored food to avoid going outside but everyone lost weight, he said.
As the town's remaining residents emerged from their homes on Monday, neighbours greeted each other for the first time in many days.
Thousands of civilians, including many from villages further south who had been forced to serve as human shields for the jihadists, escaped to government camps over the weekend while others were forced deeper into Islamic State-held territory.
"If the forces had come just a few days later, we would be in Mosul now. Daesh wanted to take us," Mr Ahmed said.
Others were not so lucky.
Tariq, an engineering student, said he had barricaded himself inside his home with dozens of neighbours for four days before Islamic State fled, refusing fighters' demands to leave with them.
At one point, he said, the fighters had donned army fatigues and managed to trick a few families into believing they were arriving Iraqi forces. When the civilians went out to greet them, they were executed, Tariq said.
"Even a one-year-old baby, they put a bullet in his head."