WASHINGTON -- The National Transportation Safety Board on Thursday criticized the government's certification process for the troubled Boeing 787   and said it had pinpointed how a battery fire occurred on one of the planes.

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 (IW 1000/16), 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."