The U.N. mission in South Sudan says a preliminary investigation shows its helicopter that crashed in South Sudan last month was shot down after a rebel commander alleged that U.N. aircraft were being used to transport government troops.

The mission said Tuesday that although Peter Gadet had threatened to shoot down U.N. aircraft, it was still premature to say who was responsible. It said Gadet made the allegation, denied by the U.N., in a phone call with a mission staffer days before the aircraft was shot down.

Three Russian crewmen were killed on Aug. 26 when the helicopter crashed near the disputed town of Bentiu in Unity state.

The mission said a technical investigation will conclusively determine the source of the ground fire which brought down the helicopter.