The first report on last month’s Jeju Air crash in South Korea has confirmed traces of bird strikes in the plane’s engines, though officials haven’t determined the cause of the accident that killed all but two of the 181 people on board.

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.

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,