South Korean police said on Thursday they had raided Jeju Air (089590.KS) and the operator of Muan International Airport as part of their investigation into Sunday’s crash that killed 179 people in the worst aviation disaster on the country’s soil.

Jeju Air 7C2216, which departed the Thai capital of Bangkok for Muan in southwestern South Korea, belly-landed and overshot the regional airport’s runway, exploding into flames after hitting an embankment.

Two crew members, who were sitting in the tail end of the Boeing 737-800, were pulled alive by rescuers but injured.

Police investigators are searching the offices of the airport operator and the transportation ministry aviation authority in the southwestern city of Muan, as the well as office of Jeju Air in Seoul, the South Jeolla provincial police said in a media statement.

Investigators plan to seize documents and materials related to the operation and maintenance of the aircraft as well as the operation of airport facilities, a police official told Reuters.

A Jeju Air spokesperson said the airline is checking the situation. The airport operator company was not immediately available for comment.

Questions by air safety experts on what led to the deadly explosion have focused on the embankment designed to prop up navigation equipment that they said are too rigid and too close to the end of the runway.

“This rigid structure proved catastrophic when the skidding aircraft made impact,” said Najmedin Meshkati, an engineering professor at the University of Southern California, adding it was concerning that the navigation antenna was mounted on “such a formidable concrete structure, rather than the standard metal tower/pylon installation”.

A probe into the Jeju Air flight is also under way involving South Korean officials and the US National Transportation Safety Board (NTSB), Federal Aviation Administration (FAA) and the aircraft’s maker, Boeing (BA.N).

It remains unanswered why the aircraft did not deploy its landing gear and what led the pilot to apparently rush into a second attempt at landing after telling air traffic control the plane had suffered a bird strike and declaring an emergency.

The aircraft’s flight data recorder, which sustained some damage, is being taken to the United States for analysis in cooperation with the NTSB.

The conversion of data from the cockpit voice recorder to audio file should be completed by Friday, acting President Choi Sang-mok said, which could provide critical information on the final minutes of the doomed flight.

A transport ministry official said on Wednesday it may be difficult to release the audio files to the public as they will be critical to the ongoing investigation.

Choi said in a disaster management meeting immediate action must be taken if a special inspection of all Boeing 737-800 aircraft operated in the country finds any issues.

“As there’s great public concern about the same aircraft model involved in the accident, the transport ministry and relevant organisations must conduct a thorough inspection of operation maintenance, education, and training,” Choi said.

Choi’s comments at the start of the meeting were provided by his office.

Investigators from the NTSB, FAA and Boeing are in South Korea to help the probe.

Choi asked that no effort be spared in helping the families of the victims as the remains of those killed are handed over them. He also asked the police to take action against anyone posting “malicious” messages and fake news on social media related to the disaster.