(CNN) -- A fire that broke out Friday in an empty Boeing 787 Dreamliner at London's Heathrow Airport does not appear to be related to the jet's batteries, the UK Air Accidents Investigation Branch said Saturday.
"It is clear that this heat damage is remote from the area in which the aircraft main and APU batteries are located and at this stage there is no evidence of a direct causal relationship," the agency said in a statement.
No injuries were reported, but the cause of the fire inside the Ethiopian Airlines plane was not clear.
The incident puts Boeing's marquee 787 Dreamliner fleet back in the headlines after it was grounded this year because of a fire risk associated with the planes' batteries.
The U.S. National Transportation Board said Saturday that it sent an investigator to London to aid in the investigation. Senior Air Safety Investigator Lorenda Ward was to be accompanied by NTSB airplane systems investigators and representatives of the Federal Aviation Administration and Boeing.
Ethiopian Airlines said in a posting on its Facebook page that the incident occurred on one of its jets after it arrived Friday morning on a normal flight from Addis Ababa to Heathrow.
After passengers deplaned and the plane was cleaned, it was towed to a remote area, where it was parked and its internal and external power switched off, Ethiopian said.
"After more than 8 hours smoke was detected," it added.
The Air Accidents Investigation Branch said no one was aboard the plane when the fire broke out, causing "extensive heat damage" to the upper portion of the rear fuselage. It predicted the initial investigation would take several days.
Another 787 Dreamliner flying out of the United Kingdom and operated by Thomson Airways was forced to turn back Friday after what the airline called "a technical issue."
The 787 Dreamliner represents a new generation of efficient wide-body, long-range aircraft, and Boeing has placed a big bet on the fleet.
Customer demand for the Dreamliner has remained solid despite the grounding. The plane is more efficient than earlier models due to its lighter weight, using composite material rather than aluminum and wires instead of some of the hydraulics.
Although Boeing has delivered to customers just 68 of the planes, it has 862 unfilled orders. Boeing says it plans to increase production to 10 per month by the end of the year, double the rate earlier this year.
The first of the long-delayed Dreamliners went into service for Japan's All Nippon Airways in October 2011, and the planes flew without major problems for more than a year. But after the battery problem emerged in January, the company had to retrofit the 50 Dreamliners already in service before they could return to service.
Though Boeing shares sank Friday 3.6% on the news, they were still up more than 30% since the start of 2013.