South Korean media say police have searched the offices of Jeju Air and Muan International Airport as part of their investigation into Sunday's deadly plane crash. The accident killed 179 people. Two crewmembers were rescued.

The searches reportedly included the Seoul office of the plane's operator as well as the airport's office and air traffic control tower on Thursday.

The media say police are investigating the accident on charges of professional negligence resulting in death. They say documents and materials related to the operation and maintenance of the aircraft have been seized.

Reports also say that Jeju Air CEO Kim E-bae has been banned from leaving South Korea as a key witness.

Aviation experts say damage to the aircraft was amplified as it hit a concrete structure after belly-landing. The structure stood about 250 meters from the end of the runway as a foundation for a landing guidance device called a localizer.

Police are looking into whether there were any issues with the airport's equipment and the flight management system.

South Korean transport ministry officials are also seeking to determine the cause of the crash in tandem with US aviation accident investigators.

The ministry said on Thursday that it had completed converting data from the plane's cockpit voice recorder into an audio file. It plans to have the retrieved flight recorder analyzed in the United States.