Correspondence obtained by Al Jazeera and The Intercept paints a bleak picture of life in ISIL-held parts of Iraq.
The Iraqi military has found about 100 decapitated bodies in a mass grave south of the ISIL-held city of Mosul.
Brigadier-General Yahya Rasool, spokesman for the Joint Military Command, said the bodies were discovered on Monday near the town of Hammam al-Alil.
Most were reduced to skeletons, he told The Associated Press news agency. The grave was found about 14km from the southern outskirts of Mosul.
The Joint Operations Command said “Iraqi forces found … 100 bodies of citizens with their heads cut off”.
AFP news agency quoted a police statement saying: “Federal police found a mass grave west of Hamam al-Alil in the agricultural college.” It did not elaborate.
A forensics team from Baghdad will investigate the site on Tuesday. Rasool said the state of the bodies made it difficult to tell by their clothes if they were soldiers or civilians.
ISIL has carried out several massacres since it swept into northern and central Iraq in the summer of 2014, often documenting them with photos and videos circulated online.
Iraqi forces launched a massive operation last month to drive ISIL fighters from Mosul and surrounding areas.
The United Nations warned two weeks ago the armed group appeared to be have killed dozens of civilians since Iraqi forces launched the offensive against Mosul.
Aid organizations, local officials, and Mosul residents have cited reports that ISIL executed 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.
Abdul Rahman al-Waggaa, a member of the Nineveh provincial council, told Reuters news agency last month most of the victims were former police and army members.
ISIL had used the town’s agricultural college as “a killing field” for hundreds of people in the days before the Iraqi government advance, 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 (IEDs) on Monday, preventing Reuters from visiting.