NTSB chairwoman Deborah Hersman said investigators probing the January 7 incident on a Japan Airlines 787 Dreamliner found evidence pointing to a single cell on the battery that caught fire on the plane parked in Boston.
There were multiple signs of short-circuiting in the cell, one of eight on the lithium-ion battery, which led to an uncontrollable rise in temperatures, or thermal runaway, to adjacent cells, she said.
"We are now working to identify the cause of the short circuit on cell six," she said at a news conference.
"We have not reached any conclusions at this point."
The battery fire, and a burned battery that forced an All Nippon Airways 787 flight into an emergency landing on January 16, resulted in the global grounding of all 50 787s in service until the problem is fixed.
As the groundings entered a fourth week, Hersman said NTSB investigators were examining the design, manufacturing and charging of the batteries.
She meanwhile faulted the battery's certification by the Federal Aviation Administration (FAA), which cleared the all-new 787 as a safe aircraft more than two years ago after extensive testing.
"The assumptions used to certify the battery must be reconsidered," she said, adding that her agency was conducting tests to determine "why hazards were not mitigated."
As part of the normal FAA certification process, which focuses solely on safety, the FAA relied heavily on information provided by Boeing, she said.
Boeing had estimated that a battery smoke event would occur less than once in 10 million flight hours, while it has instead happened twice in less than 100,000 flight hours, she said.
"We have seen two events on two aircraft less than two weeks apart," she said.
Transportation Secretary Ray LaHood, in a statement following the NTSB news conference, said the FAA, which is also investigating the two burned battery incidents, would take "any action necessary to further ensure safety."
"We must finish this work before reaching conclusions about what changes or improvements the FAA should make going forward," LaHood said.
Boeing welcomed the "progress" reported by the NTSB in the probe and reiterated it was assisting in the NTSB and the FAA investigations.
"We are working collaboratively to address questions about our testing and compliance with certification standards, and we will not hesitate to make changes that lead to improved testing processes and products," the Chicago-based company said in a statement.
Meanwhile, the Federal Aviation Administration approved Boeing's request to conduct test flights of its 787 Dreamliner aircraft as part of the FAA's battery fire probe.
"These test flights will be an important part of our efforts to ensure the safety of passengers and return these aircraft to service," the FAA said.
Hersman said the NTSB had completed disassembly of the burned battery and was wrapping up the "microscopic" investigation, while continuing to work closely with Japanese and French counterparts.
French firm Thales designed the Dreamliner's electrical system and commissioned Japanese firm GS Yuasa to produce the batteries.
A detailed NTSB report on the two-track probe -- investigation of the fire and a review of the battery certification and testing -- will be issued within 30 days, Hersman said.
Since the global grounding, the US aerospace giant has halted deliveries of the 787, which began service in October 2011 as an energy-efficient aircraft making extensive use of lightweight composite materials and pioneering electrical systems.
Boeing has continued a production pace of five 787s a month.
Richard Aboulafia, an aerospace industry analyst at Teal Group, predicted the prolonged battery investigation could result in the need for "re-certification," keeping the 787 grounded for six months or longer.
Dow member Boeing's shares closed up 1.5 percent at $77.43 Thursday.