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 aboard 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 from Boeing, have been combing the crash site since the disaster to establish a cause, with both black boxes found and decoding work ongoing.

“In relation to the plane accident … a search and seizure operation is being conducted from 9 am (5:00am PST) on January 2 at three locations,” police said in a statement sent to AFP.

“The police plan to swiftly and rigorously determine the cause and responsibility for this accident in accordance with the law and principles.”

Police told AFP that 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 Sunday crash.

South Korea’s acting president, Choi Sang-mok, said 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.

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

“If any issues are found during the inspection, please take immediate corrective action,” he added.

The accident is the worst-ever aviation disaster on South Korean soil.

South Korean authorities have completed the initial extraction of data for the cockpit voice recorder, but the flight data recorder was damaged and was to be sent to the United States for analysis, officials said Wednesday.

‘I miss you’

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 travelled 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’s belongings.

On Thursday at the airport, a monk led prayers close to the crash site, while inside the airport, the stairs were covered in colourful post-it notes left by mourners. “Honey, I miss you way too much,” one of them said.

“Even if you faced lonely and painful moments in death, may you now soar like a butterfly,” another read.

Relatives also left flowers and food near the site including tteokguk — rice cake soup traditionally enjoyed in South Korea on New Year’s Day — as they said their goodbyes, many in tears.

Local media reported on a handwritten note, seemingly written by the pilot’s brother, that was placed alongside gimbap — a popular Korean dish — and a drink cup near the crash site.

“I feel heartbroken when I think about the struggles you faced alone (in your last moments),” it read.

“You were truly amazing and did so well, so I hope you can now find happiness in a warm place. Thank you, and I’m sorry.”

On Wednesday — a public holiday in the South — hundreds of people queued up to pay their respects at an airport memorial altar set up to honour the victims.

So many people came to the memorial that the queue stretched for hundreds of metres and the local cell phone network was overloaded, local media reported. Other altars for the victims have been set up nationwide.