South Korea’s transport ministry has extended by a week special inspections of all 101 of the Boeing 737-800 jets run by the country’s airlines, as jet engine maker joined a probe into the deadliest aviation disaster on the country’s soil.

The ministry launched the inspections following Sunday’s crash of a Boeing 737-800 operated by Jeju Air that killed 179 people.

The inspections were supposed to be completed on Friday but were extended to Jan. 10 for additional checks, such as whether airlines spent enough time carrying out maintenance and secured parts for repairs, a ministry official told reporters.

The Jeju Air flight from Bangkok to Muan county in southwestern South Korea belly-landed and overshot the regional airport’s runway, exploding into flames after hitting an embankment.

The ministry has said it would look at engines, maintenance records and landing gear on all 737-800s, and an airline’s operations could be suspended for serious violations.

The plane’s engines are produced under GE’s CFM International joint venture with Safran.

While it is unclear yet what caused the disaster, the Jeju Air crash adds to headaches faced by Boeing as the planemaker battles to restore trust with customers following two fatal 737 MAX crashes, a mid-air panel blowout, and a seven-week strike.

The transport ministry also held an emergency meeting with the chief executives of 11 airlines, including top-ranked Korean Air Lines and Asiana Airlines to discuss measures to enhance aviation safety.

South Korea’s investigation team said on Friday two of its members would leave for the United States next week to analyse the flight data recorder of the crash in cooperation with the U.S. National Transportation Safety Board (NTSB).

The team is also studying the plane wreckage and interviewing airport control tower officials.

Investigators will analyse data on 107 mobile phones recovered from the crash site, including text messages, for clues on what happened leading up to the crash, Yonhap News said.

South Korean acting President Choi Sang-mok on Friday urged investigators to work swiftly to collect evidence from the crash scene and analyse a voice recorder.

Unanswered questions include 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.

Police said on Thursday they were searching Jeju Air and the operator of Muan International Airport and banned Jeju Air chief executive Kim E-bae and an unidentified official from leaving the country.