French anti-terrorist prosecutors announced Monday an investigation of an attack on a sightseeing excursion in Niger, West Africa that left six French citizens dead.

Axar.az reports that, the deadly rampage Sunday was carried out by men on motorcycles armed with rifles who appeared by surprise out of the bush. Most of the French victims were employees of ACTED, a French humanitarian group.

Two Nigerians were also killed in the attack: the guide, who led the local Association of Giraffe Guides, and the driver of the excursion vehicle.

The attack took place when the group was out to view local giraffes around the town of Koure, 60 kilometers (37 miles) east of Niemey, the country’s capital.