The search area is 6 times the size of the United States.
Upon hearing the plane no debris found, no flash in the sky along the waypoints, ATC and ADS-B transponders turned off, neither the Flightware nor FlightRadar24 have tracks of the flight, Chinese passengers' smartphones seen by relatives still online at QQ, that means something unusual out of ordinary.
Based in the above, obviously the plane might have been hijacked, people commandeering the plane know what they're doing but they didn't know ACARS, avionics stuff pilots don't always know about nor they can turn off, also enabled in the plane.