The first report on last month’s Jeju Air crash in South Korea confirmed the plane’s two black boxes stopped recording about four minutes prior to the crash that killed 179 people on board.

The preliminary accident report released by South Korea’s Aviation and Railway Accident Investigation Board on Monday said the flight data and cockpit voice recorders on the Boeing 737-800 had stopped working, confirming what the country’s Transportation Ministry initially said earlier this month. 

South Korean officials had sent the devices to the U.S. National Transportation Safety Board for closer examination after discovering that some of the data was missing. It remained unclear why the devices stopped recording.

The report also found traces of bird strikes – feathers and bird blood stains – in both the plane’s engines, though officials have yet to determine what caused the crash.

“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.

Investigators earlier said that air traffic controllers warned the pilots about possible bird strikes two minutes before the aircraft issued a distress signal confirming that a bird strike had occurred, after which the pilots 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 had filmed the plane coming close to birds during an aborted landing.

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 to ultimately determine the cause of the crash.

Officials said the report has been sent to the International Civil Aviation Organization, Thailand, the U.S. and France, adding that the aircraft was built in the U.S. and its engines in France.

The Associated Press contributed to this report.