Britain’s Air Accidents Investigation Branch (AAIB) classified the fire that started on a Boeing 787 Dreamliner at Heathrow Airport in London on Friday as a “serious incident.”

The “serious incident” classification is a step below an “accident” on the investigator’s scale for categorizing cases, according to a Reuters report.

The fire started on the Ethiopian Airlines plane while it was parked at Heathrow, eight hours after it arrived from Addis Ababa. There were no passengers on the plane and no one was injured. Both of Heathrow’s runways were shut down for more than an hour due to the incident, but no flights were affected Saturday.

The cause of the fire has not been determined.

“There has been extensive heat damage in the upper portion of the rear fuselage, a complex part of the aircraft, and the initial investigation is likely to take several days,” the AAIB said in a statement.

The AAIB has not found evidence that the fire was caused by problems with the plane’s batteries. Boeing’s Dreamliners were out of commission for three months earlier this year because of battery issues.

The National Transportation Safety Board (NTSB), USA, representing the State of Design and Manufacture, and the Civil Aviation Authority of Ethiopia, representing the State of Registry and Operator, were invited to appoint representatives to participate in the investigation, according to a statement by the AAIB. Advisors from the Federal Aviation Administration, Boeing Commercial Airplanes and Ethiopian Airlines will also participate.

Boeing addressed the incident on Twitter on Friday.

Our team is on ground working in support of UK authorities & Ethiopian Airlines to determine cause of 787 event @HeathrowAirport.

— Boeing Airplanes (@BoeingAirplanes) July 12, 2013

The aircraft is being stored in a hangar at Heathrow while the technical investigation into the cause of the fire is conducted.