The National Transportation and Safety Board has found signs of short-circuiting and thermal runaway in the lithium-ion battery that caught fire Jan. 7 on a Japan Airlines Boeing 787 on the ground at Boston Logan International Airport. But the safety board has not yet found any indications that the battery was overcharged nor has it come close to determining another cause.

“The work that we continue to do will tell us why these things happened,” NTSB Chairman Deborah Hersman said during a briefing Jan. 24 on the investigation’s progress.

A thermal runaway is an increase in temperature that changes conditions in a way that causes additional increases in temperature—in this case an uncontrolled chemical reaction between electrolyte and electrode. It can be caused by factors such as a short-circuit or collateral damage from one of the battery’s eight cells that short-circuited or overheated, an NTSB official says. But the safety board’s examination of the damaged battery has not yet determined the sequence of events, or whether the damage originated from the battery or was caused by an associated system, such as a charger or auxiliary power unit.

Hersman’s remarks suggested there still is a lot of work to be done to reach the point of finding answers, absent a sudden breakthrough. “We have not ruled anything out,” she said. “We are still evaluating all of the potential failure scenarios.”

The data that the NTSB has been able to pull from the flight data recorder do not show any charge exceeding the battery’s 32.2-volt maximum. But investigators still are trying to corroborate that finding, both for the entire battery and the individual cells.

“We do not have any data that shows the battery was overcharged,” Hersman added.

The NTSB’s investigation is a bit ahead of a similar investigation by the Japan Transport Safety Board of an inflight incident concerning the lithium-ion battery Jan. 16 on an All Nippon Airways 787. Because of that, Hersman said the investigators have not yet been able to determine whether the damage to the two batteries was the same.

“We’re looking forward to being able to compare,” she said.