PARIS (AP) — French anti-terrorism prosecutors formally opened an investigation one day after seven aid workers, most of them French citizens, and a leading guide were killed by gunmen in a Niger giraffe park considered one of the safer places in the West African country.

The group had been on a day of sightseeing Sunday when they were attacked just before noon, according to one of the aid groups they worked for, Paris-based NGO ACTED. An ambulance sent by the French military stationed in Niger found the bodies later in the day in their burned-out vehicle in the giraffe reserve in Koure, the group said.

The victims were six French citizens and one Nigerien working for ACTED and Geneva-based IMPACT Initiatives. The other victim was their Nigerien guide. Four of the French aid workers were women, two men, and all were between the ages of 25 and 50, according to ACTED.

The NGOs condemned “in the strongest terms the senseless and barbaric killing of our colleagues and their guide.”

French prosecutors said in a statement Monday they opened an investigation for “murders in relation to a terrorist undertaking.”

Niger’s interior ministry confirmed Sunday the attack took place at the giraffe reserve in Koure. Hundreds of people visit each year the protected national park that contains dense vegetation and tall trees about 70 kilometers (45 miles) southeast of the capital.

“Our colleagues have been working to support the people of Niger facing hardship, driven by values of humanity and solidarity,” the NGOs said.

The Association of Giraffe Guides of Koure also released a statement condemning the death of the president of their association, Kadri Abdou.

“We are deeply saddened and thinking of the victims and their families to whom we offer our most sincere condolences and especially to the family of Kadri, our friend. May he rest in peace,” said the statement.

Niger President Issoufou Mahamadou also condemned the killings in a post on Twitter.

“I condemn the cowardly and barbaric terrorist attack perpetrated this Sunday in the peaceful town of Koure,” he wrote, extending condolences to the families of the victims and to the French president, adding that France's “commitment to our side in the fight against terrorism is unwavering.”

One of the biggest French NGOs, ACTED has been present in Niger since 2010 and provides aid to displaced people and local populations who are particularly vulnerable due to conflicts in the region, lack of food and droughts.

A partner to ACTED, IMPACT Initiatives was first deployed in Niger in 2012. It conducts mapping programs and other projects in camps hosting displaced populations.

In a phone call on Sunday, French President Emmanuel Macron and Niger's president pledged to clarify the circumstances of the deadly attack by “all means,” the French presidency said.

French Foreign Affairs minister Jean-Yves Le Drian condemned the attack on Monday, saying that those responsible “will have to answer for their acts.”

The French government has warned citizens against traveling outside Niger's capital, Niamey, as militants linked to Boko Haram, Islamic State and al-Qaida carry out attacks across the vast West African nation. Niger borders several countries including Mali, Burkina Faso, Chad, Nigeria and Libya.

Violence by rebels linked to the IS group and al-Qaida is on the rise in the Sahel region. France has deployed 5,100 soldiers to help fight the growing insurgency there, and a local Sahel force made up of soldiers from Mali, Niger, Burkina Faso, Chad and Mauritania has also been fighting the extremists.

ACTED said that it is now barring its staff across Niger, Chad, Mali and Burkina Faso from leaving their zones as a result of the attack.

The group insisted that it had taken necessary security measures, requiring clearance three days ahead of time for their staff to travel within Niger. ACTED co-founder Marie-Pierre Caley said it was the first such attack in the area of the giraffe park, and described it as much safer than other areas in Niger where the group's staffers work.

ACTED co-founder Frederic Roussel said it may have been an opportunistic attack by a small group targeting Westerners, but said it was too early to say for sure.

He described increasingly tight security measures for ACTED workers in recent years, but said the attack shouldn't deter aid workers from “helping the most threatened populations on the planet.”

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Petesch reported from Dakar, Senegal.