Officials in Japan have performed CT scans on faulty Boeing Dreamliner batteries but remain uncertain if the cells failed because of an electrical or chemical cause.
Investigators may now widen their tests to other equipment on the technologically advanced aircraft, Japan Transport Safety Board (JTSB) chairman Norihiro Goto said.
The batteries, from a 787 operated by All Nippon Airways (ANA), overheated last month and forced the plane to make an emergency landing.
Investigators in Japan and the US continue to examine two incidents with the plane's lithium-ion batteries - a battery fire on a Japan Airlines (JAL) 787 at Boston airport and the emergency landing on the domestic ANA flight after battery problems triggered a smoke alarm.
Another aircraft suffered a cracked cockpit window on a domestic flight in Japan.
Mr Goto said CT scans showed six of the main battery's eight cells on the ANA Dreamliner were badly damaged, charred and deformed.
The news comes as Boeing (NYSE: BA - news) has asked the US Federal Aviation Authority (FAA) for permission to undertake test flights on the troubled model, which has been grounded since the incidents.
He said Boeing had not discussed its test flight request with the Japanese agency, and he did not know if the plane maker had found clues to the cause of the battery problems.
Approaching the FAA for test flights "could mean they have made progress,"the JTSB spokesman said.
ANA is the world's biggest Dreamliner operator with 17 of the jets while local rival JAL owns seven.
Japan accounts for almost half the 50 787s that have been grounded since January 17.
Around a third of the composite construction aircraft's components were sourced in Japan by Boeing.
Last Friday, US officials said they were making progress in their investigation into the battery fire on JAL's jet in Boston, although they have yet to set any timetable for completing their work.
ANA said it lost around £11m in revenue as a result of the Dreamliner grounding, while JAL said the halting of 787 flights would shave £5m from its operating profit in the year to the end ofMarch.
Both companies have said they will discuss compensation for the losses with Boeing.
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