Though the fuselage was nearly destroyed by the intensity of the fire, the
cockpit voice recorder (CVR) and
flight data recorder (FDR) for flight 797 were still in good condition and produced vital data for the NTSB investigation.
[3] On the CVR, NTSB investigators heard eight sounds of electrical arcing—likely inaudible to the crew—beginning at 18:48 CDT. Three minutes later, at 18:51, the popping sounds that Cameron and Ouimet would later identify as the left toilet circuit breaker tripping are audible on the CVR; Cameron attempts to reset the circuits twice over the next 60 seconds, but the CVR records the breakers immediately popping again after each reset attempt.
[4] Cameron would attempt once more to reset the breakers at 18:59, but the CVR records arcing sounds followed by the popping sound of the breakers continuing to trip again after each reset over the next 60 seconds.
[4] At 19:02, the CVR records flight attendant Judi Davidson entering the cockpit to deliver the first report of a possible fire in the lavatory.
[4] Though a number of wires in the lavatory section were later found with insulation stripped away, NTSB investigators were unable to determine whether this insulation damage was the cause of the fire or was caused by the fire.
This from the NTSB report. Cause "undetermined" ? More like can't tell which wire bundle went first. And continual resetting of the tripped circuit breaker was not unrelated, either.