Anatomy of a missile strike - from the report on MH 17, downed over Ukraine:
Damage observed on the forward fuselage and cockpit section of the aircraft appears to indicate that there were impacts from a large number of high-energy objects from outside the plane.
The pattern of damage observed in the forward fuselage and cockpit section of the aircraft was not consistent with the damage that would be expected from any known failure mode of the aircraft, its engines or systems.
The fact that there were many pieces of aircraft structure distributed over a large area, indicated that the aircraft broke up in the air.
That's technical speak for "a missile hit the plane," which of course we already knew. Full report (pdf) here.