Although many of the victims of the attacks this year have been Hema, experts say this new wave of violence does not follow the usual pattern of ethnic killings and reprisals.

“It’s not an ethnic conflict in the sense that these are not people who have always hated each other and are just going to kill and maim and rape each other because one of them is Lendu and one of them is Hema,” Ms. Autesserre said.

In some cases, the attackers are men who survivors say were speaking languages from other regions. In other cases, the attackers are Lendu, but no one knows what their motives are — and no one knows who is supporting them.

Many of the attacks were coordinated and carried out with new weapons and expensive communications equipment, suggesting the fighters have powerful backers who may be looking to exploit the animosity between the two ethnic groups for their own purposes.