South Korean officials confirmed Saturday that the black boxes of the passenger jetliner that crashed in South Korea last year, killing 179 passengers, stopped recording four minutes before impact.

The South Korean Transportation Ministry reported that after analyzing both devices, the U.S. National Transportation Safety Board determined that the cockpit voice recorder and flight data recorder stopped working four minutes before the crash.

After its landing gear failed, the Boeing 737-800 operated Jeju Air skidded from a runway on December 29 in South Korea’s town of Muan, slamming against a concrete building and bursting into fire, killing all 181 passengers on board except two.

The NTSB was also contacted by South Korean officials after they discovered that some data had been lost.

It is not immediately apparent why the devices did not record data for the last four minutes, according to the Ministry of Transportation.

The ministry stated that data from the CVR and FDR are vital in investigating accidents. However, such investigations are conducted by examining and analyzing various sources of information. We plan to do everything we can to determine the reason for the accident.

Investigators in South Korea have stated that air traffic controllers had warned the pilot of possible bird strikes about two minutes before he sent a distress signal to confirm that the bird strike occurred. After that, the pilot tried an emergency landing.

After the crash, the authorities ordered an immediate inspection of all 737-8 aircraft operated by the airlines in the country – dozens total – following the accident.

South Korean officials also promised to improve airport security after experts linked the high death rate to Muan Airport’s localizer, which was the structure that the aircraft hit as it crashed. Localizers are antennas that guide aircraft when landing. They were housed inside a concrete structure, covered in dirt, on an elevated embankment. The structure was made of concrete, which is heavier than other materials and would have broken more easily when struck.