If the transponder is working you would, I believe, be able to track it all the way down depending upon how often it squawks. If the power is cut or there is some catastrophic failure (boom!) then probably not.
If that is true, then clearly on its face this plane “ended” in the air. And no one wants to say it out of respect for the families, I suppose?
Egypt air board member on CNN reporting that there were three security personnel on board.