NTSB Chairman Deborah Hersman said a full forensic investigation is under way into the incidents, which she called "a very serious safety concern."
Boeing's fleet of Dreamliners -- its newest and most advanced plane -- was grounded by the Federal Aviation Administration by a series of electrical fires related to its lithium-ion batteries.
In two of the latest incidents, a fire broke out in a Japan Airlines 787 Dreamliner at Boston's Logan Airport, after passengers had deplaned. Another incident caused an All Nippon Airways Dreamliner to make an emergency landing in Japan.
As part of the investigation, the NTSB disassembled the batteries and determined they had not exceeded their designed voltage of 32 volts. However, Hersman said, it appeared the batteries had short-circuited in at least the Boston case, The Hill reported.
"We know the lithium ion battery experienced a thermal runaway, we know there were short circuits, and we know there was a fire," Hersman said, cautioning, "We are not determining the cause of the event, just sharing some characteristics.
"One of these events alone is serious; two of them in close proximity, especially in an airplane model with only about 100,000 flight hours, underscores the importance of getting to the root cause of these incidents," Hersman said.
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