Airliner flies along. Suddenly transponder is switched off/fails at time x, likely causes .. Interference//tech issue//sudden destruction of aircraft, allied with comms possibilities as follows..
A/Radio transmission from flight ends abruptly. Likely cause..explosion/or/explosive decompression. Sudden destruction.
B/No radio calls from crew. Likely cause..Crew wouldn't have Known the transponder had failed and continued.
C/ATC tried to contact the crew in range and there was no response. Likely causes...destruction at x/or/interference at x/or/radios made u/s by same tech issue as transponder eg. Fire//electrical problem.
Mu!tiple eyewitnesses report an unusual large aircraft flying fast and low. Coupled with there being no evidence of sudden destruction, either in the form of debris, infrared flash monitoring, seismic registration or eyewiynesses in a very densely populated area points to either interference or tech issue, fire or electrical. As the eyewitnesses have mentioned that the aircraft was carrying lights, it couldn't have been a total Electrical failure. (If this large aircraft flying in an unusual direction at an unusual altitude on the very night this 777 goes missing is indeed the 777) . if it wasn't a total electrical failure then it would have been almost certainly possible to get either the transponder or com1 or acars or HF going.
Which points to two things. If it was a fire, it was now under control or it would have Been out of control by this time, but wasn't, or the bits would have been found by now. The witnesses specifically mention white light, not fire. If the fire had been contained I daresay the pilots would have landed it asap.
So it wasn't a fire.
Which leaves us with the last cause... Interference. If the aircraft was seized the scenario would fit the facts..seizer/s turned off the transponder, prevented the Crew from transmitting, and forced them to fly somewhere else, or flew themselves, deviating from the flight plan. Where they were seen by eyewitnesses. Or painted by primary radar.
We know two things by deduction..the aircraft was unlawfully taken control of and By now the aircraft is either crashed or landed safely somewhere in the hands of the criminals.