I'm reading that they might suspect that the pilots or one of the pilots, switched off the transponder manually just before the plane vanished. If so, that could indicate that the person deliberately tried to erase all traces of his actions prior to the plane crashing into the sea, which is a pattern seen previously when pilots have taken drastic measures to commit suicide, attempting to mask the event as an accident in the process.
If that theory is being investigated, that could explain why they are stonewalling the public. It's very bad pr if people fear that travelling with this airline, is like playing a game of Russian Roulette with your life.