Air accident investigators say they have ruled out the involvement of lithium-ion batteries in a fire onboard an Ethiopian Airlines Boeing 787 at London Heathrow Airport on July 12.

Heathrow closed both its runways as firefighters tackled the blaze in the rear fuselage of the aircraft as it was parked on a remote stand at the western end of the airport. Television pictures showed burn marks on the fuselage just forward of the tail fin.

In a statement on July 13, the U.K. Air Accident Investigation Branch (AAIB) reported that it had begun an investigation and this would likely take several days.

“It is clear that this heat damage is remote from the area in which the aircraft main and APU (Auxiliary Power Unit) batteries are located, and, at this stage, there is no evidence of a direct causal relationship,” says the AAIB.

Investigators say the fire had resulted in smoke throughout the fuselage and “extensive heat damage” in the upper portion of the rear fuselage. The aircraft has been moved to a hangar at Heathrow where investigators are conducting their work.

The AAIB says it has invited representatives from the U.S. NTSB and the Ethiopian civil aviation authority to participate in the investigation.

The aircraft involved was ET-AOP (L/N44), which was the first 787 to restart operations in mid-April following the fleet-wide grounding related to battery problems.

The incident forced Heathrow to be temporarily closed, and flights were diverted to Luton, Stansted and Gatwick airports while emergency services tackled the blaze. News of the incident prompted Boeing’s share price to temporarily tumble 7%.