transponder goes silent. >> when you turn that off, it's like lights are out. so there's no more voice communication. >> and no signal to air traffic control with the plane's location, speed and altitude. >> it could have been physically turned off. there could have been a circuit breaker pulled. or there could have been a catastrophic failure on the plane, which caused an electrical disruption. >> then the 777 makes an unexpected left turn, heading quest and way off course. >> what we see is a quick turn, almost 180 degree turn back toward land. >> according to a source close to the investigation, military radar then shows the plane dropping and flying as low as 12,000 feet. >> the controller at the other end is expecting a call from that aircraft. >> but the call never comes.