Unknown assailants killed at least three forest guards in Democratic Republic of the Congonear a national park that is home to nearly half of the world’s mountain gorillas, an endangered species, and where attacks by armed groups are frequent.

An engineer is missing and three other rangers were injured in the attack, which took place after the vehicles of the Congolese Institute for Conservation of Nature (ICCN) left the village Kiwandiya, in the province of North Kivu. Dozens of armed, paramilitary organizations operate in this area.

“We don’t know who the perpetrators are,” said the district commander, Alain Cueva. The ICCN did not respond to a request for comment on this information.

The ICCN is responsible for the protection of the neighbor Virunga National Park. Last February the Institute warned of a resurgence of violence in the region after Mai Mai separatist rebels attacked a ranger outpost in Virunga, killing one man and injuring two others.