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

Authorities have retrieved the black boxes from the wreckage, but extracting information from the flight data recorder (FDR) is proving difficult. It reportedly sustained some damage during the crash, and it has been decided that sending it to the United States is the best course of action.

The Boeing 737-800's damaged flight data recorder (FDR) was sent to the National Transportation Safety Board (NTSB) in Washington for data extraction. The cockpit voice recorder data was extracted in Seoul, but when investigators realized some data was apparently missing, they sent the device to the NTSB for further analysis.