So, we don't know yet when the last ACARS transmission was received? And how this correlates with the last transponder contact? Or the last 'ET calling home' message from the engines?
Presumably the Malaysian authorities must have access to all relevant information. If all sources of communication stopped at about the same time, the most likely reason is that the aircraft is in the sea near the last known transponder position. If not, it suggests that something odd was going on.
Is this why they're searching to the west of the Malaysian peninsula? Because they have reason to think the plane was still active after the transponder stopped?