Investigations says at least 500 people were killed, including families burnt alive in their homes and a two-year-old who was thrown into a septic tank.
Ethnic violence broke out when one community wanted to bury one of their traditional chiefs on another community’s land.
The investigation adds that violence could flare up again at any time.
Investigators say the violence between 16 and 18 December was “planned and executed with the support of customary chiefs”.
Members of the Batende community attacked Banunu villages “with extreme violence and speed, allowing little time to escape