WASHINGTON -- U.S. government investigators said Thursday they could not yet explain what caused a potentially catastrophic battery fire aboard a Boeing 787 that forced the grounding of Dreamliners around the world.

National Transportation Safety Board Chairwoman Deborah Hersman said investigators were working "around the clock" to understand what led to the Jan. 7 fire aboard a parked Japan Airlines 787, as well as a smoking battery that forced the emergency landing of an All Nippon Airways 787 on Jan. 16.

But she said that the systems designed to contain such an event on the all-new Dreamliner "did not work as intended."

"This is an unprecedented event. We are very concerned. We do not expect to see fire events on board aircraft," Hersman told reporters.

"The expectation in aviation is to never experience a fire on board an aircraft. In two weeks' time we saw two cases of battery failures on a 787 and the grounding of the entire fleet by the FAA. The significance of these events cannot be understated."

Hersman said the NTSB had still not reached any conclusions on what caused the Jan. 7 battery fire aboard the empty plane at Logan airport in Boston.

When investigators first saw it after the fire was extinguished, she said, there was structural and component damage around the battery, that specific plane's auxiliary power unit (APU).

"The APU battery was spewing molten electrolyte, very hot material," she said.