A series of problems with Boeing's next-generation aircraft sparked multiple probes around the world and the grounding of the entire Dreamliner fleet last month after the domestic All Nippon Airways (ANA) flight was forced to land.
Japanese investigators have focused on the ANA Dreamliner's main lithium-ion battery, which was severely damaged by what they believe to have been a build up of heat that resulted in uncontrollably high temperatures.
Japan's Transport Safety Board said Wednesday it had discovered that the circuit wiring of the burned battery and another one were connected, even though this is not typical in airplanes.
However, investigators added that the battery system has a system to block a reverse current and it had remained intact so the "unusual" wiring was an unlikely culprit for the overheating.
Investigators said they would now expand the scope of their search to include the aircraft's circuit wiring.
"We have been investigating what happened, but as we haven't got to a point where we can say why it happened yet, we can't say we've made progress," Norihiro Goto, chairman of the Japan Transport Safety Board, told a press briefing.
The unusual circuit wiring may have affected the digital flight data recorder's measuring of voltage in the burned battery, investigators added.
ANA and Japan Airlines (JAL), major customers of the aircraft with more than 100 combined orders, have been hit hard by the grounding, slashing hundreds of flights, affecting tens of thousands of passengers.