To this point, aircraft parts have been found scattered over a 6-mile distance. To me, this suggests that the 737, which had been cruising at 29,000 ft, broke up in midair with parts falling over a wide distance. Additionally, the reports so far indicate that the descent was virtually an immediate straight-down descent, with some sort of momentary partial recovery several thousand feet from the ground, followed by a resumption of the vertical descent. If that is accurate, to me it would suggest that the wings were still attached to the aircraft at the point of the momentary recovery, and that the pilots had at least some type of partial control, at least for a short time.
If the above is true, then pilot error as a cause would almost certainly be eliminated.
Aircraft incidents of this nature are generally caused by either catastrophic failure of a major aircraft component, or by an on-board explosion of some type. A possible scenario could be a catastrophic failure of one engine, damaging either the wing or some other vital aircraft components. However, given the long and excellent performance of this 737-800 model, and the fact that this aircraft was only some five years old, that scenario may be questionable. The engine maintenance history will be of extreme importance in considering the situation.
I would be inclined to consider the explosion scenario. If that should prove to be the case, it will be very interesting to see how forthcoming the Chinese air authorities prove to be with respect to accurate information.
One flight recorder, believed to be the cockpit audio recorder, has been recovered.