Four American special operations soldiers were killed over the weekend when their aircraft crashed in Djibouti, in northeast Africa, as it returned from a mission.

The soldiers were riding in an U-28, a single engine prop plane used by Special Forces. The plane crashed on Saturday around 8 p.m. near Camp Lemonnier, a military base in the Horn of Africa. The plane was reportedly returning from a mission in support of the war in Afghanistan.

The cause of the crash is still under investigation. However, initial indications do not appear that the plane was under attack at the time of the incident. A statement from U.S. Africa Command said it was a routine flight, reported the Associated Press.

The military announced the four soldiers killed soldiers were Capt. Ryan P. Hall, 30, of Colorado Springs, Colo., with the 319th Special Operations Squadron; Capt. Nicholas S. Whitlock, 29, of Newnan, Ga., with the 34th Special Operations Squadron; 1st Lt. Justin J. Wilkens, 26, of Bend, Ore., with the 34th Special Operations Squadron, and Senior Airman Julian S. Scholten, 26, of Upper Marlboro, Md., with the 25th Intelligence Squadron, according to The Associated Press.

Hall was a U-28 pilot who had logged more than 1,300 combat flight hours.