The US National Transportation Safety Board (NTSB) provided a detailed description of the Jan. 7 Boeing 787 fire in Boston, but offered no conclusion regarding the incident’s root cause.

In an interim factual report issued Thursday, available here, NTSB gave an extensive description of the fire aboard a Japan Airlines 787-8 parked at Boston Logan Airport following a flight from Tokyo. But it said it has not determined the cause of the fire, which the board believes was sparked when short-circuiting occurred in a cell of the lithium ion battery used to start the aircraft’s auxiliary power unit. The short circuit led to thermal runaway—uncontrolled chemical reactions resulting from overheating—that spread quickly throughout the 32-volt battery.

The report described how difficult it was for firefighters to put out the fire, which “appeared to be rekindling” even as measures were being taken to control it. A firefighter “reported that the battery was emitting white smoke, creating heavy smoke conditions,” NTSB stated, adding that the firefighter “also reported that the battery was hissing loudly and that liquid was flowing down the sides of the battery case.”

One firefighter “received a burn on his neck when the battery, in his words, ‘exploded,’” NTSB said.

The board said it will continue a two-track investigation into the Boston 787 fire. One track seeks to find “the probable cause of the 787 battery fire,” NTSB stated. The second track is reviewing “the design, certification, and manufacturing processes for the 787 lithium ion battery system.”

NTSB noted that Boeing’s pre-certification analysis of lithium ion battery cell failure modes “determined that overcharging was the only known failure mode that could result in [a battery] cell venting with fire.” However, it has been determined by NTSB that overcharging did not occur in the Japan Airlines 787 fire. Similarly, the Japan Transport Safety Board has said overcharging did not occur in the All Nippon Airways Jan. 16 lithium ion battery overheating event that led to an emergency landing.

NTSB will hold two 787-related public hearings in April.