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.

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

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.

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.