"The body of evidence strongly suggests that the event initiated in cell number six," Deborah Hersman, chairman of the National Transportation Safety Board, told reporters Thursday in Washington, D.C.
However, the NTSB has not determined what caused the battery cell to short-circuit. Hersman said the agency is still considering a number of causes.
"Design, certification, manufacturing: These are all still on the table," she said. "We have a lot of work to do."
Meanwhile, one 787 did fly on Thursday. Boeing received permission from the Federal Aviation Administration to fly one from Fort Worth, Texas, to Everett. The permission was for a single flight to relocate the plane, and was not a test flight, the FAA said in a statement.
The Federal Aviation Administration grounded Boeing 787s on Jan. 16 after a second 787 experienced a battery failure. That jet, which was operated by All Nippon Airways, made an emergency landing in Japan on Jan. 15.
Hersman said the NTSB is looking at the FAA's certification process of Boeing's 787, including the special conditions under which it approved the use of the jet's lithium-ion battery.
"The assumptions used to certify the battery must be reconsidered," she said.
Several of those assumptions have proved inaccurate. For one, Boeing believed it unlikely that a fire that started in a single cell would propagate to any of the other seven. The company also assumed that a battery failure, like the ones on Jan. 7 and Jan. 16, might occur only once every 10 million flight hours.
The 787 fleet has less than 100,000 flight hours when the two failures occurred.
"We know that some of the assumptions that were made ... were not met," she said.
Hersman previously called the 787 battery incidents a "serious safety concern."
John Goglia, a former NTSB board member and aviation safety expert, said NTSB's findings mean the government will likely require Boeing to re-certify the batteries.
"Certifications aren't exactly painless and quick," he said. "It could be a big, drawn-out thing."
The significance of the NTSB's findings "is if this can happen — and the safety analysis assumed that it would not happen — then the safety analysis is no longer valid," said Jon Hansman, a Massachusetts Institute of Technology aeronautics professor and a member of the FAA's Research and Development Advisory Committee.
Battery experts said Boeing could try to build more safeguards into the battery by using a greater number of smaller cells and putting more insulation between them. Or, they said, the aircraft maker could switch to a different type of lithium ion battery already approved for aviation.
Switching to another type of battery, such as lead-acid or nickel-cadmium battery, is another possibility, but that would involve changing the charging system as well, they said, and add weight. The new batteries — and, presumably, a revised charging system — would need to be designed and tested by Boeing and approved by the FAA before they could be installed.
In a statement released Thursday, Boeing said it's committed to working with the NTSB and FAA in the investigation. The company's 787 was certified after a "rigorous" test program, Boeing said.
"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," Boeing said in its statement.
The decision to resume 787 flights is the FAA's. The NTSB is investigative and has no regulatory authority, but its recommendations have influence.
FAA Administrator Michael Huerta and Department of Transportation Secretary Ray LaHood issued a joint statement on the 787 investigation Thursday.
"From day one, we have said that the comprehensive review of the Boeing 787 and the root cause analysis of the two battery incidents would be a data-driven process," LaHood and Huerta said in their statement. "Based on what information our experts find, the FAA will take any action necessary to further ensure safety."
Prior to grounding the 787, the FAA launched a comprehensive review of the Dreamliner's design, manufacture and assembly. That review continues; and the NTSB has been invited to observe in the process, LaHood and Huerta said Thursday.
Earlier this week, Boeing asked the FAA for permission to resume test flights of the 787 to try out solutions. The Wall Street Journal reported Wednesday that Boeing is looking to redesign the battery to have more separation between battery cells.
The FAA has only approved the single 787 flight to bring a jet in Texas back to Everett.
The Associated Press contributed to this report.