Conclusions are being drawn by a second team of investigators regarding the crash, and I quote from
http://news.bbc.co.uk/2/hi/europe/8130989.stm"an examination of the recovered wreckage led them to believe the plane probably hit the water "in the direction of flight and with a strong vertical acceleration".
BBC transport correspondent Tom Symonds said if the plane had broken up in the air, pieces of the fuselage would have been found twisted in a variety of directions.
Instead they showed signs of compression in one direction, resulting from the plane hitting the water on its belly.
Life jackets found in the wreckage had not been inflated, indicating that the passengers had little warning of a water landing."
So, rather than a mid-air breakup, according to this report, it is most probable that the plane was descending extremely rapidly immediately before the crash, belly down, before impacting, so that the passengers did not have time to react, to inflate life jackets, etc.