The Department of Transport is investigating the cause of data loss following the worst aviation disaster in the country’s history.

Black box that holds flight data and cockpit voice recorder Jeju Air’s Boeing 737-800 jet South Korea’s Transport Ministry said recording stopped about four minutes before a plane crash in South Korea last December.

On December 29, while flying from Thailand to Muan International Airport in South Korea, Jeju Air flight 7C2216 landed on its belly, hit a concrete barrier and exploded. 179 of 181 passengers and crew died.

This is the worst air crash ever on South Korean soil.

“Analysis shows that neither CVR nor FDR data were recorded in the four minutes before the aircraft collided with the localizer,” South Korea’s Transport Ministry said Saturday of the two recording devices.

The locator, an obstacle at the end of the runway that helps planes land, has been blamed for exacerbating the severity of the crash.

The recorder was initially analyzed in South Korea and was sent to a U.S. National Transportation Safety Board laboratory when data was found to be missing, the ministry said.

But a box holding clues to the flight’s final moments appears to have lost data, and authorities are trying to figure out what happened.

“Plans are in place to investigate the cause of the data loss during the ongoing investigation into the incident,” the ministry said.

Former Transport Ministry accident investigator Sim Jai-dong told Reuters that the discovery of the missing data was surprising and suggested that all power on the plane, including backup power, may have been cut off, which is rare.

Investigators said the boxes were critical to their investigation, but added they would not give up their efforts to find the cause of the crash.

Investigators pointed to possible bird strikes, landing gear failure and runway obstructions.

The pilot also warned of a bird strike before exiting the first landing and initiating a go-around.

But rather than conducting a comprehensive exploration, Boeing 737-800 jet aircraft Make a sharp turn, approach the airport’s single runway from the other end, and make an emergency landing without deploying the landing gear.

This week, lead investigator Lee Seung-yeol told reporters that “feathers were found” in one of the plane’s recovered engines, but warned that the bird strike would not immediately cause engine failure.

Authorities raided offices at Muan Airport where the crash occurred, a regional aviation office in the southwestern city and Jeju Air’s offices in the capital Seoul.

They also Jeju Air CEO banned from leaving the country.

Transport Minister Park Sang-woo offered to resign earlier this week as the investigation continued, saying he felt a “heavy sense of responsibility for this tragedy.”