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 Boeing 737-800 was flying from Thailand to Muan in South Korea on 29 December when it crash-landed and exploded into a fireball after slamming into a concrete barrier.

It was the worst aviation disaster on South Korean soil, killing 179 of the 181 passengers and crew.

South Korean and American investigators are still probing the cause of the disaster, with a bird strike, faulty landing gear and the runway barrier among the possible issues.

Both engines recovered from the crash site were inspected, and bird bloodstains and feathers were “found on each,” the report said.

“The pilots identified a group of birds while approaching runway 01, and a security camera filmed HL8088 coming close to a group of birds during a go-around,” the report added, referring to the Jeju jet’s registration number.

It did not specify whether the engines had stopped working in the moments leading up to the crash.

DNA analysis identified the feathers and blood as coming from Baikal teals, migratory ducks which fly to Korea in winter from their breeding grounds in Siberia.