The first report on last month’s Jeju Air crash in South Korea has confirmed traces of bird strikes in the plane’s engines, though officials haven’t determined the cause of the accident that killed all but two of the 181 people on board.

An Airbus plane belonging to South Korean carrier Air Busan caught fire on Tuesday at Gimhae International Airport in the country’s south while preparing for departure to Hong Kong, fire authorities said.

The exact cause of the Jeju Air crash remains unclear, and the investigation is complicated because the black boxes stopped recording four minutes before impact.