The Japan Transport Safety Board's probe into the emergency landing of an All Nippon Airways Co. Boeing 787 said no faults were found in the aircraft's electrical system after a lithium-ion battery overheated under the cockpit.

The board said Wednesday there might have been a defect either inside the Dreamliner's battery or in the way it was connected to its charger.

"When the problem occurred, there was no external flow of current from the battery and it was only connected to the battery charger," Norihiro Goto, the safety board chairman, told reporters in Tokyo. "We can only think of two possibilities — that (there might have been a fault) either inside the battery or in its connection to the charger."

The board also said that about 10 holes apparently caused by sparks were detected in the stainless-steel battery case, which melts at around 1,400 degrees.

Boeing's 787s have been grounded worldwide after the ANA Dreamliner made an emergency landing in Kagawa Prefecture on Jan. 16 due to smoke in the cockpit. The lithium-ion battery under the cockpit was later found to be severely damaged and charred.