French and Nigerien soldiers searched through a giraffe reserve and its surrounding area in Niger's Koure region on Monday for the killers of six French aid workers.
The gunmen, who arrived on motorcycles, also killed the workers' guide and driver, bringing the death toll to eight.
The aid workers were attacked Sunday as they drove through the giraffe reserve, a popular tourist spot for expatriates living in Niamey, the capital of Niger, located about 50 kilometers from Koure.
"This heinous crime must not go unpunished, nor will it distract us from our commitment to support the people of Niger," ACTED, the non-governmental organization that employed the aid workers, said in a statement.
ACTED director and co-founder Marie-Pierre Caley said in a statement that the group followed security protocol by reporting their arrival at the reserve after departing from Niamey on Sunday.
Internal army memos seen by Reuters appear to show the attack occurring less than an hour after their arrival.
According to ACTED, four men and four women, all between the ages of 25 and 50, died during the attack.
The region was not viewed as unsafe prior to the attack.
"Until yesterday, the area had not known any security incident," said ACTED development director Frederic Roussel.
No group has claimed responsibility for the attack, although militant violence by terrorist organizations such as Boko Haram has been on the rise in Niger.
France's office of anti-terrorism has launched an investigation.
French President Emmanuel Macron tweeted, "Our determination to combat armed terrorist groups is resolute. The fight continues."
