Duck remains were found in both engines of the Jeju passenger jet that crashed last month, killing 179 people in the worst air disaster in South Korea’s history.

The first preliminary report released on Monday suggests a combination of a bird strike and the concrete structure at the end of the runway were behind the deadly incident.

The exact cause of the crash remains unclear and the investigation is ongoing.

The feathers and blood stains found in the engines were from the Baikal teal, a migratory duck that flies to South Korea for winter in large flocks, according to investigators.

The six-page report gave no indication about what might have led the Boeing 737-800 jet to land far down the runway without its landing gear deployed, highlighting a lack of clues.

The investigation has been complicated by the discovery that the plane’s two black boxes stopped recording four minutes before impact.