"These are individual graves, but behind us, by the trees, are the mass graves of those executed by Daesh (IS)," said Asaad Mohammad, the 56-year-old forensic assistant at the site.

"There are some 2,500-3,000 bodies estimated there, plus between 900 and 1,100 bodies in the individual graves, so at least 3,500 total."

According to scripture they're not allowed to bury muslims in mass graves. So the 3000 bodies in the mass grave are not considered muslims, which means they are definitely executed by IS.

It also means the other 1,100 in the individual graves are more likely from:

Victims of air strikes

Victims of different rebel groups who occupied the town

Victims of SAA

Someone correct me if I'm wrong?