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.

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.

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,