South Korea’s authorities investigating last month’s Jeju Air plane crash have submitted a preliminary accident report to the UN aviation agency and to the authorities of the United States, France and Thailand,

The exact cause of the Jeju Air crash remains unclear, and the investigation is complicated because the black boxes stopped recording four minutes before impact.

A preliminary report on the Jeju Air plane that crashed in South Korea in December killing 179 on board reveals evidence of a bird strike. The report says that both engines of the plane contained duck remains but it provided no conclusions about what caused the plane to land without its landing gear deployed or why flight data stopped recording in the final four minutes of the flight.