WASHINGTON -- The Boeing 787 Dreamliner battery that caught fire earlier this month in Boston shows evidence of short-circuiting and a chemical reaction known as "thermal runaway," in which an increase in temperature causes progressively hotter temperatures, federal accident investigators said Thursday.It's not clear to investigators which came first, the short-circuiting or the thermal runaway, National Transportation Safety Board Chairman Deborah Hersman said. Nor is it clear yet what caused either of them, she said during a news briefing.The fire took place aboard a Japan Airlines 787 shortly after it landed at Logan International Airport on Jan. 7. All the passengers had left the craft, but a cleaning crew noticed smoke in the cabin 26 minutes after the plane arrived at its gate. It took firefighters nearly 40 minutes to put out a battery fire in the aircraft's rear auxiliary power unit.Investigators are still dissecting the charred insides of the battery at the board's laboratory in Washington in an effort to piece together clues to the root cause of the fire.A little over a week after the fire in Boston, another 787 battery failure led to an emergency landing in Japan. There were no flames in that incident, but there was smoke in the cabin, Hersman said."The expectation in aviation is to never experience a fire on board an aircraft," yet there were two battery failures on the 787 within two weeks, Hersman said. "We have to understand why this battery resulted in a fire when there were so many protections that were to be designed into the system."After the battery fire in Boston, the Federal Aviation Administration ordered a review, and on Jan. 16, the agency grounded the six U.S. 787s operated by United Airlines.