Flags flew at half-mast on Monday as South Korea mourned 179 people killed in the worst plane crash on its soil, as investigators probe why the Jeju Air plane crash-landed and burst into flames.The country has started seven days of national mourning,

South Korean officials are struggling to determine what caused a deadly plane crash that killed nearly 180 people on board, with the nation saddened, shocked and ashamed over the country’s worst aviation disaster in decades.

Footage of the crash aired by YTN television showed the Jeju Air plane skidding across the airstrip and colliding head-on with a concrete wall on the outskirts of the facility.