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.

A preliminary report published on Monday, Jan. 27, said feathers and blood stains were found in both engines of the Boeing 737-800 that crashed at Muan International Airport airport on Dec. 29, according to CNN and the BBC.