Thirteen French soldiers fighting Islamist extremists in Mali have been killed after two military helicopters collided in mid-air. The deadly accident happened late Monday during a combat operation in the West African nation. French President Emmanuel Macron expressed “deep sadness” at the news Tuesday, paying tribute in a statement to the “courage of the French soldiers” fighting in the Sahel region. French Defense Minister Florence Parly said an investigation has been opened to how the catastrophe was allowed to happen. The operation in West and Central Africa is France’s largest overseas military mission and involves round 4,500 personnel, the Associated Press reports. Before his death last month, ISIS leader Abu Bakr al-Baghdadi praised “brothers” in Mali and neighboring Burkina Faso for pledging their allegiance.