A preliminary investigation into last month’s fatal cargo plane crash at Hong Kong International Airport has found that one of the aircraft’s engines unexpectedly accelerated after landing, authorities said Tuesday.

The Boeing 747, operated by Turkey-based ACT Airlines on a flight from Dubai, skidded off the runway on October 20 before striking a security patrol car and plunging both vehicles into the sea. Two airport security workers inside the car were killed, while all four crew members on board the aircraft escaped unharmed.

According to the Transport and Logistics Bureau, initial evidence suggested that weather conditions, runway surface and air traffic control operations were all normal before the aircraft veered off course.

A report released by the Air Accident Investigation Authority classified the event as an accident. Investigators said the aircraft’s Number 4 engine was found accelerating even after touchdown.

An inspection of the cockpit showed that thrust levers for engines one, two and three had been closed, with reverse thrust fully selected. However, the Number 4 engine thrust lever was discovered in the full forward position, with its reverse thrust lever also pushed fully forward.

Authorities said the probe will now focus on determining why the Number 4 thrust lever was positioned that way, and whether mechanical issues or loss of control contributed to the incident.

Investigators will collect additional data and analyse engine performance, aircraft systems, maintenance history and possible human-factor considerations. A full report is expected within a year.

The investigation is being supported by experts from Turkey’s Transport Safety Investigation Center, the U.S. National Transportation Safety Board and Boeing. The aircraft was operating under lease for Emirates, the Dubai-based long-haul carrier.