The verified deaths of at least 535 men, women and children in fighting between two communities in Democratic Republic of Congo in December may amount to crimes against humanity, the UN human rights office has said.
The UN said in January there were credible reports of at least 890 deaths in the clashes over three days in four villages in Yumbi territory in the west of the country.
It sent a special investigative mission that found inter-communal attacks had been planned and carried out.
The fighting between Banunu and Batende communities was triggered by a dispute over the burial of a Banunu chief, the investigators found.
Batende villagers attacked with extreme violence and speed, allowing little time to escape, the UN said.
"In some cases, witnesses report that victims were asked if they were Banunu, before they were killed. Many were killed as they tried to cross the Congo River. Others were burnt alive in their homes," a UN statement said.
The report details horrors documented such as a two-year-old reportedly thrown into a septic tank.
More than 19,000 people were displaced, and almost 1,000 buildings, including churches, schools and health centres, were destroyed or looted.
The report said the violence was facilitated by a lack of intervention by Congolese provincial authorities, who "appear to have failed in their responsibility to protect the population".
"The violence could flare up again at any time", the investigators' report said.
The investigators only managed to reach three out of the four locations and the casualty numbers were likely to be higher than the verified figure because bodies were believed to have been thrown into the Congo River.
The violence was spread over three days in four villages in Yumbi territory in the west of the country.
Batende villagers attacked with extreme violence and speed in a planned action, decapitating and maiming people who had little time to escape, the investigators said.
The UN said in January there were credible reports of at least 890 deaths in the violence. The new report verified 530 Banunu men, women and children dead and just five Batende.
The violence could explode again at any time as there were areas where the two communities lived together, UN human rights spokeswoman Ravina Shamdasani said.
The violence was facilitated by the failure of Congolese provincial authorities to intervene, the report said.
Police had left before the attacks took place, despite indications of rising tension. But there was no evidence of complicity, Ms Shamdasani said, although some police officers had taken part in the attacks in a personal capacity.