Authorities investigating the deadly Jeju Air passenger plane crash last month said the flight data recorder and cockpit voice recorder have no data for the flight’s final four minutes.

According to the Aviation and Railway Accident Investigation Board on Saturday, the black box stopped recording four minutes before the collision.

The conclusion was based on an investigation conducted by the U.S. National Transportation Safety Board at the request of the accident investigation board.

The reason for the missing data remains unknown.

The accident investigation board vowed to do its utmost to determine the cause of the accident, saying that while the flight data recorder and cockpit voice recorder are important, the investigation also involves the analysis of other data.