An investigation into a deadly cargo plane crash in Hong Kong last month found an engine of the aircraft accelerated after touching down, according to a preliminary report released on Tuesday.

The Boeing 747, flown by Turkey-based ACT Airlines from Dubai, skidded after landing on October 20 and collided with a security patrol car, sending both vehicles into the sea. Two workers in the car were killed. The four crew members on the plane were unhurt.

The Hong Kong Transport and Logistics Bureau wrote in a Facebook post that existing evidence showed the flight and conditions such as weather, runway and air traffic control were all normal before the plane veered off.

The preliminary report from the Air Accident Investigation Authority said the plane’s Number 4 engine accelerated after landing.

Examination of the flight deck showed thrust levers of Number 1, 2 and 3 engines were closed and their reverse thrust levers were selected to the maximum.

“The Number 4 engine thrust lever was in the full forward thrust position. Number 4 engine reverse thrust lever was fully forward,” the report said.

The bureau said the investigation would focus on why Number 4 engine’s thrust lever was in that position, including whether there were problems that caused its loss of control.

The investigation authority will collect further data and conduct analysis focusing on issues such as those related to aircraft systems and performance, conditions of the engines, maintenance records and human factors.

The bureau said the investigative agency aims to finish a comprehensive report within a year. It added the authority was assisted by representatives from Turkey’s Transport Safety Investigation Center and U.S. National Transportation Safety Board, in addition to Boeing’s experts.

The aircraft was operated under lease by Emirates, a long-haul carrier based in Dubai.