U.S. planemaker Boeing, desperate to reassure customers that its flagship Dreamliner plane is safe, may have to wait several days for the first signs of what caused a fire at London's Heathrow airport on Friday.

Britain's Air Accident Investigations Branch (AAIB) is leading the probe into the blaze on the Ethiopian Airlines jet and has already allayed fears about a return of problems with overheating batteries that grounded the Dreamliner for months earlier this year.

But a source close to the investigation said it would likely be days before the agency was able to draw conclusions on the cause, and some industry analysts said experience suggested an initial report might only come at the end of this week.

"The investigation is still at an early stage and no final report on it is imminent," said the source, speaking on condition of anonymity because he is not authorised to speak publicly on the probe.

"The next step is likely to be publication of a full report on the incident or a safety bulletin if the AAIB feel they need to alert the industry to widespread fault that could be systemic."

A 25 strong team of experts, including inspectors from the AAIB and the U.S. National Transportation Safety Board are investigating the damaged Dreamliner in a hangar at Heathrow airport, some 15 miles west of central London.

A source close to Boeing said the company had officials "on the ground" at Heathrow but that the AAIB-led team were "operating on their own timescale" and had not provided details of when any further public statements would be made.

Such investigations typically take between a week and two weeks to draw conclusions, said Paul Hayes, director of safety at the London-based Ascend aviation consultancy. Boeing would be anxious for the cause and origin of the fire, which caused extensive damage to the upper portion of the jet's rear fuselage, to be released faster than that to quell speculation among airline customers and investors.

"Boeing will want the final bulletin out as soon as possible to stop all the waffle and provide certainty. Given the public interest in it I'd expect the AAIB to get it out quickly, probably in the next couple of days," said Ascend's Hayes.

"It could easily have been a case of human error or a short-circuit in a plug and that's what Boeing will be hoping."

Boeing declined to comment on the state of the investigation when contacted by Reuters.