Investigators probing a Jeju Air crash that killed 179 people in the worst aviation disaster in South Korea said Wednesday they will send one of the retrieved black boxes to the United States for analysis.

The Boeing 737-800 was carrying 181 people from Thailand to South Korea Sunday when it issued a mayday call and belly-landed before hitting a barrier and bursting into flames, killing everyone aboard except two flight attendants pulled from the burning wreckage.

South Korean and U.S. investigators, including from Boeing, have been combing the crash site in southwestern Muan since the disaster to establish a cause.