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First off that means that the initial search location was hundreds of miles from where the plane was last identified.
To visualize what happened, the plan takes off from Kuala Lumpur, gets to the edge of Vietnamese airspace toward Ho Chi Minh City. It then turns around,flies back over the ocean to Malaysia and get all the way to the Strait on the other side of the country (demarked here by the redline.)
The reports suggest that even after the transponders were turned off, civilian and military radar in the region continued to track the plane. So continued tracking but with much less detailed information and no positive ID about the identity of the plane.
On its face, when I first heard that, it seemed like the only possible explanation was some form of hijacking, even if the 'hijacker' was one of the pilots or people charged with flying the plane. Why else do you turn off your transponders?
But experts note that a complete loss of electrical power could account for the transponders going off.
Basically it's totally unclear what happened. But pilots who are discussing this on the cable nets point to another possible scenario: If the plane experienced a total loss of electrical power, that would turn off the transponders. But it would also allow the plane to keep flying, at least for some period of time.
So perhaps the plane experienced electrical failure, knocking out all communications and telemetry from the plane, the pilots turn around but somehow get lost and the plane eventually goes down.
Again, to be clear, that the hypothetical sketched out by a highly experienced pilot. But purely a hypothetical based on an almost unprecedented and hard to interpret collection of data.