A conservation group says suspected rebels in Congo killed a wildlife ranger returning from a mission to track endangered gorillas.

The New York-based Wildlife Conservation Society, or WCS, said Friday that the ranger was fatally shot a day earlier in Kahuzi-Biega National Park in eastern Congo, where many rebel groups operate.

WCS representative Andrew Plumptre says the park guard was killed while investigating a truck that had been looted and left on the side of a park road.

The Wildlife Conservation Society supports efforts to protect Grauer's gorillas, the biggest of the different gorilla types, in Kahuzi-Biega park. The gorilla population is under pressure from illegal hunting and mining activity that is shrinking its habitat.

Rangers in other Congolese parks, including Garamba and Virunga, also face threats from armed groups.