Seoul. The first report on last month’s Jeju Air crash in South Korea confirmed traces of bird strikes in the plane's engines, though officials haven’t determined the cause of the accident that killed all but two of the 181 people on board.

The preliminary accident report released by South Korea's Aviation and Railway Accident Investigation Board on Monday said that feathers and bird blood stains were found in both engines.

“The samples were sent to specialized organizations for DNA analysis, and a domestic organization identified them as belonging to Baikal teals,” the report said, referring to a migratory duck.

The report also said the plane's two black boxes — the flight data recorder and cockpit voice recorder — stopped recording about 4 minutes before the crash. This could complicate efforts to find the cause of the crash.

The Boeing 737-800 skidded off the runaway at Muan International Airport on Dec. 29 after its landing gear failed to deploy, slamming into a concrete structure and bursting into flames. The flight was returning from Bangkok and all of the victims were South Koreans except for two Thai nationals.

Many analysts said the concrete structure, which housed a set of antennas called a localizer that guides aircraft during landings, should have been built with lighter materials that could break more easily upon impact. South Korea's Transport Ministry announced last week that it will remove the concrete structure at the airport.

Investigators earlier said that air traffic controllers warned the pilot about possible bird strikes two minutes before the aircraft issued a distress signal confirming that a bird strike had occurred, after which the pilot attempted an emergency landing.

The preliminary report said the pilots also noticed a group of birds while approaching the runway at the Muan airport and that a security camera filmed the plane coming close to birds during an aborted landing as well.

The report said authorities will disassemble the engines, examine their components in depth, analyze the black box and air traffic control data, and investigate the embankment, localizer and bird strike evidence.

“These all-out investigation activities aim to accurately determine the cause of the accident,” the report said.

The Transport Ministry said the preliminary report has been sent to the International Civil Aviation Organization, Thailand, the United States and France. It said the plane was built in the U.S. and its engines in France.

It said the Muan airport will remain closed until April 18.