Both engines of the Jeju Air plane that crashed last month contained duck remains, according to a preliminary report on Monday, with authorities still trying to determine what caused the deadliest air disaster on South Korean soil.

The six-page report released by South Korean authorities a month after the crash said both engines of the Boeing 737-800 jet contained DNA from Baikal Teals, a type of migratory duck that flies to South Korea for winter in huge flocks.

But the report provided no initial conclusions about what may have caused the plane to land without its landing gear deployed, and why flight data recorders stopped recording in the final four minutes of the flight.