LONDON -- A fire aboard an empty 787 at Heathrow Airport spooked Boeing investors Friday, as they feared the re-emergence of battery problems that grounded the plane for months earlier this year.

Boeing shares lost $5.01, or 4.7 percent, to $101.87.

The cause of the fire on the Ethiopian Airlines plane -- which broke out more than 8 hours after it landed in London -- remained under investigation. The location of the fire led experts to surmise it wasn't the plane's lithium-ion batteries.

Runways at Heathrow were shut down for nearly an hour as emergency crews put out the fire. No passengers were on board.

The 787, which Boeing dubs the Dreamliner, was grounded in January following two incidents with its lithium-ion batteries. One 787 caught fire after it landed at Boston's Logan International Airport on Jan. 7.

Boeing marketed the plane as a revolutionary jet which -- thanks to its lightweight design -- burns 20 percent less fuel to comparable aircraft. Boeing, based in Chicago, has delivered 66 of the planes to customers with another 864 on order.

Photos showed the rear roof of the plane burned. The batteries are located in two compartments under the floor of the plane. One is near the wings; the other under the cockpit. Friday's fire wasn't near either of those areas.

Unanswered questions kept Boeing stock sharply lower. Spokesman Marc Birtel said in an email that the company "is working to fully understand and address" the situation.

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