The human rights group Amnesty International says satellite images and witnesses point to possible mass graves in Burundi that appeared after security forces retaliated against rebel gunmen in Bujumbura.
"These images suggest a deliberate effort by authorities to cover up the extent of the killings by their security forces and to prevent the full truth from coming out," Muthoni Wanyeki, Amnesty's director for East Africa, said Thursday.
Amnesty said police and security forces killed dozens of people on December 11 after rebels attacked three military bases in the capital.
The group said police stormed into homes and dragged out men, killing some on the spot. It said that while some of those shot may have taken part in the rebel raid, many did not.
At least 87 people were killed, with some sources putting the death toll much higher.
Amnesty said that while bodies were everywhere, almost none were brought to nearby morgues, with witnesses saying many bodies were dumped into at least five mass graves.
The United Nations also is investigating. There has been no comment from the Burundian government.
Political violence has raged in Burundi since President Pierre Nkurunziza announced last April that he was going to run for a third term. The opposition called that unconstitutional and said it broke the terms of a peace agreement to end the country's long civil war.
Nkurunziza was re-elected in July, and there are no signs of the violence easing.
The African Union has said it fears Burundi could sink into genocide.