Bird feathers and bloodstains were found in both engines of the Jeju Air plane that crashed in December, according to a preliminary investigation released Monday.

South Korea released on Monday a preliminary report on its investigation into a Jeju Air crash on Dec. 29 at Muan International Airport that killed 179 people in the deadliest air disaster on the country's soil.

The preliminary report was released by the Aviation and Railway Accident Investigation Board on Monday in South Korea.

Investigators found bird feathers and blood in both engines of the Jeju Air jet that crashed in South Korea last month, killing 179 people, a person familiar with the probe said on Friday. The Boeing 737-800 plane,

The investigation into the deadliest air disaster on the country's soil remains ongoing, focusing on the role of bird strike and involving an analysis of the engines and the "localizer" landing guidance structure.

Authorities have suggested that migratory bird strikes were the cause of last month's deadly plane crash in South Korea, according to multiple reports.

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,