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.

A preliminary report says evidence of a bird strike was found on the Jeju Air plane that crashed into a wall at a South Korean airport, killing 179 people.

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.