The investigation into the Jeju Air crash has hit a major roadblock after it was revealed that the aircraft’s black box did not record the final four minutes leading up to its collision with a localizer at Muan International Airport.

The missing data deepens the puzzle of what caused the deadly air disaster in Muan, South Korea, late last month.

After initial analysis of the devices, the US National Transportation Safety Board concluded that both the flight data and cockpit voice recorders stopped working about four minutes before the crash