A fire initiated in the top portion of the JB-1 junction box, which contains components associated with the distribution of electrical power from the APU or an external AC power supply. The fire ignited combustible materials, including insulation blankets and spread upwards toward a flexible oxygen line mounted above the JB-1 junction box. The flexible oxygen line ignited when exposed to the fire, and the fire burned through the aircraft’s fuselage. Evidence suggests that the fire initiated as a result of an electrical anomaly in the top portion of the JB-1 junction box, near bus bar and contactor components. This evidence includes melting of one side of the bus bar along the periphery of its connection to a terminal stud and severe thermal damage to the internal and, to a lesser degree, external surfaces of the K1XB contactor case. Despite the damage to the contactor case, the internal contacts did not appear to be stuck or exhibit signs of melting or excessive pitting. There were no other signs of arcing near any of these components. Numerous maintenance difficulties had occurred on the aircraft involving the external AC power supply system beginning approximately 10-weeks before the accident. To attempt to remedy the anomalies, several relays, electrical contactors, the AC power switch, the external ground power receptacle, and other electrical components were removed and replaced with new components. Several of these components were located in the upper section of the JB-1 junction box. The last of these actions occurred about 5-weeks before the accident. Probable Cause: An electrical anomaly in the top portion of the JB-1 junction box resulting in ignition of adjacent combustible materials, including insulation blankets, and a flexible oxygen line.