hurry. >> at 1:21 a.m., the plane's 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 some kind of catastrophic failure on the plane which caused an electrical disruption. >> then the 777 makes an unexpected left turn heading west 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 control at the other end