There are more questions in the disappearance of Malaysia Airlines flight 370.
Officials say the plane was hundreds of miles off course, and its transponder, which signals its location to the ground, was turned off or stopped working just as it changed course.
Tom Foreman has been looking into what might have happened.
A former Boeing 777 pilot, Mark Weiss, gives his take.
Why do airline manufactures give the option to switch off the communication systems?