NAIROBI — Four U.S. soldiers returning from a reconnaissance mission have been killed in an air crash near a U.S. military base in Djibouti in the Horn of Africa, U.S. officials said.

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The four were killed when their U-28 aircraft crashed on Saturday, a statement released by the Department of Defense late Monday said.

"The Department of Defense announced today the deaths of four airmen who were supporting Operation Enduring Freedom ... The cause of the accident is under investigation," the statement said.

Operation Enduring Freedom includes the war in Afghanistan, but also covers U.S. operations in a number of other locations including the Horn of Africa.

Hostile fire not suspected
The four killed in the crash were: Capt. Ryan P. Hall, 30, of Colorado Springs, Colorado, with the 319th Special Operations Squadron; Capt. Nicholas S. Whitlock, 29, of Newnan, Georgia, with the 34th Special Operations Squadron; 1st Lt. Justin J. Wilkens, 26, of Bend, Oregon, with the 34th Special Operations Squadron; and Senior Airman Julian S. Scholten, 26, of Upper Marlboro, Maryland, with the 25th Intelligence Squadron.

Hall was a U-28 pilot with more than 1,300 combat flight hours. He was assigned to the 319th Special Operations Squadron at Hurlburt Field, Fla.

Specialist Ryan Whitney of the 1st Special Operations Wing said that initial indications are that the plane did not crash because of hostile fire.

The plane was conducting an intelligence, surveillance and reconnaissance mission, he said. A statement from U.S. Africa Command called it a "routine" flight.

The Red Sea state also hosts the largest French army camp on the continent and is used by anti-piracy naval patrols.