JUBA, South Sudan (AP) — A report by African Union investigators says mass graves have been discovered in South Sudan and cites horrific crimes against civilians, including forced cannibalism.

The report released late Tuesday also disputes that there was a coup attempt in December 2013 by former Vice President Riek Machar.

The African Union investigators, led by former Nigerian President Olusegun Obasanjo, found that the conflict began with a skirmish between members of the presidential guard in the capital, Juba, followed by the government-organized killings of ethnic Nuer civilians and soldiers.

President Salva Kiir is a Dinka while Machar is a Nuer.

Hundreds of Nuer men were rounded up and shot. The report says Perpetrators tortured their victims, including by forcing them to eat human flesh.