MUAN, South Korea — South Korean police on Thursday raided the offices of Jeju Air and the operator of Muan International Airport as part of the probe into the fatal crash of a Boeing 737-800 plane that killed 179 people.

The flight was carrying 181 people from Thailand to South Korea on Sunday when it issued a mayday call and belly-landed before slamming into a barrier, killing everyone on the plane except two flight attendants pulled from the burning wreckage.

Authorities carried out search and seizure operations at the Muan airport where flight 2216 crashed, a regional aviation office in the southwestern city, and Jeju Air's office in the capital Seoul.

South Korean and US investigators, including those from Boeing, have been combing the crash site since the disaster to establish a cause, with both black boxes found and decoding work ongoing.

"The damaged flight data recorder has been deemed unrecoverable for data extraction domestically," said Joo Jong-wan, South Korea's deputy minister for civil aviation.

"It was agreed today to transport it to the United States for analysis in collaboration with the US National Transportation Safety Board."

Joo earlier said both of the plane's black boxes were retrieved, and for the cockpit voice recorder "the initial extraction has already been completed".

"Based on this preliminary data, we plan to start converting it into audio format," he said, meaning investigators would be able to hear the pilots' final communications.

The second black box, the flight data recorder, "was found with a missing connector", Joo said.

Police told AFP the search warrant was issued, and Yonhap reported it was approved on charges of professional negligence resulting in death, citing officials.

South Korea has also announced it will inspect all Boeing 737-800 aircraft operated by its carriers, focusing on the landing gear, which appears to have malfunctioned during the crash.

South Korea's acting President Choi Sang-mok said on Thursday that "immediate action" must be taken if that probe uncovered any issues with the aircraft model.

Authorities have previously said 101 aircraft of the same model were in operation by six different airlines.

Worst aviation disaster

The crash is the worst aviation disaster on South Korean soil.

Officials initially pointed to a bird strike as a possible cause of the disaster and have since said the probe was also examining a concrete barrier at the end of the runway.

Dramatic video showed the plane colliding with it before bursting into flames.

The land ministry said in a statement that the country's airports were also being inspected to evaluate the installation locations around their runways.

As the investigation continued, families traveled to the crash site for the first time to pay their respects on Wednesday and some returned the following day with cardboard boxes to collect their loved ones' belongings.

On Thursday at the airport, a monk led prayers close to the crash site, while inside the airport, the stairs were covered in notes left by mourners.

Questions by air safety experts on what led to the deadly explosion have focused on the embankment, designed to prop up the "localizer" antenna used to guide landings, which they said is too rigid and too close to the end of the runway.

"This rigid structure proved catastrophic when the skidding aircraft made an 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 or pylon installation".

Agencies Via Xinhua