The voice and data recorders pulled from the wreckage of the Jeju Air Co. jet in South Korea failed to capture the final moments before the Boeing Co. 737-800 made an emergency landing and crashed, depriving investigators of evidence to help understand the cause of the country’s deadliest aviation accident.

Both the flight data and cockpit voice recorders suspended recording for the final four minutes before the plane ploughed into a structure at the end of the runway, the Korean Transport ministry said in a statement on Saturday.

A joint investigation team comprising Korean and US officials is seeking to determine what caused the devices to stop working, according to the statement. 

The revelation is a setback for the investigative team trying to solve what led to the deadliest aviation disaster that killed 179 out of 181 on board. Flight data recorders, commonly referred to as black boxes, are fortified devices meant to withstand even the most severe impacts, as well as fire or extended submersion. They tape key parameters of an aircraft’s performance as well as cockpit voices and sounds that support crash experts. 

“This implies they were not receiving power,” said Darren Straker, a former head of air accident investigation units in the United Arab Emirates and Hong Kong. Straker said it indicated one possibility of a major electrical systems failure or a power supply interruption.

The Jeju Air jet heading from Bangkok to Muan International Airport in South Korea was attempting an emergency landing on December 29 when it skidded on its belly off the runway and exploded after hitting a concrete structure. The incident occurred a few minutes after the airport control tower had warned the pilot of the risk of bird strikes.

Several elements leading up to the crash remain unclear. The pilot initiated a so-called go-around after his first landing attempt, though didn’t make a full circle and instead was cleared to land in the other direction of the runway. It’s unknown why the landing gear didn’t deploy, and why the cockpit crew didn’t extend landing flaps and slats that would have helped slow down the incoming aircraft. 

One of the engines appears to have lost power in flight because of a bird strike, according to video footage, but it hasn’t been established if the other one was also shut down, either by accident or because of bird strike. Engines are the main source of power for an aircraft, though in theory there are ways to deploy the landing gear mechanically even when a plane has lost electricity.

Investigators have also focused on an installation just beyond the perimeter of the runway that proved fatal. The concrete wall was mounted with guiding instruments for incoming aircraft, and the Jeju Air plane was unable to slow down enough as it skidded down the runway and ended up slamming into the embankment, where it disintegrated.

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