South Korea released a preliminary report into last month’s deadly Jeju Air Co. crash that offers more detail on the role of bird strikes in the disaster, while still leaving crucial questions unanswered.

The feathers and blood stains on both engines of the Jeju Air plane were from the Baikal teal, a type of migratory duck that files in large flocks, according to a preliminary investigation report published on Monday.

Pilots’ actions after the bird strike are an early focus of the investigation, according to people familiar with the probe.

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,

South Korean authorities concluded search and recovery operations and an investigation into the Dec. 29 crash of a Jeju Air plane that killed 179 people. The Korea JoongAng Daily said Sunday that the families of the victims agreed to end the investigation after authorities disclosed details about bird strikes linked to the crash.

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South Korean authorities have submitted their preliminary findings on the Jeju Air crash that killed 179 people to the International Civil Aviation Organization (ICAO) and aviation authorities in the United States,

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.

The nation’s transport ministry reviewed structures near airport runways after the deadly crash of a Jeju Air flight late last month.

South Korea will extend runway safety areas and redesign infrastructure after the crash of a Jeju Air Co. flight last month that killed almost everyone on board, sparking criticism that the design of the airport might have exacerbated the accident.