– More than a dozen bodies were unearthed from a mass grave near the northern Iraqi city of Tikrit on Thursday, as a new U.N. report said militants with the Islamic State of Iraq and the Levant may have committed genocide, war crimes and crimes against humanity during their rampage across the country.

The discovery and the report’s findings — detailing mass killings, torture and rape — raised fears that more atrocities could be uncovered as Iraqi security forces and Shiite militias claw back territory from the extremist group.

Iraqi authorities in Salahuddin province unearthed 13 bodies in the district of al-Boajeel, east of the city of Tikrit, where security forces and Shiite militias are engaged in a large-scale offensive against the militants.

A government official and a senior military official said an investigation is underway to identify the dead.

Iraqi troops backed by powerful Shiite militias are currently in a holding position on the edges of Tikrit as they wait for any remaining civilians to leave before pushing toward the center of the city. ISIL captured Tikrit and Iraq’s second largest city Mosul during its rapid advance across north and west in June. The extremists now rule a vast area straddling the Syria-Iraq border.

The U.N. report draws on the testimony of 100 people who survived attacks by ISIL in Iraq between June 2014 and last month. The accounts detail killings, torture, rape and sexual slavery, forced religious conversions and the conscription of children.

Associated Press