Investigators identified evidence of arcing in wiring of the in-flight entertainment network, but this did not trip the circuit breakers, which were not designed to trip on arcing. The investigation was unable to determine whether this arc was the "lead event" that was assumed to have ignited the flammable covering on MPET insulation blankets that quickly spread across other flammable materials. After the crew cut power to "non-essential" cabin systems, a reverse flow in the cockpit ventilation ducts increased the amount of smoke reaching the flight deck. By the time the crew became aware of the severity of the fire, it had become so extensive that it was impossible to address as it happened.