The flight data recorder will give information about the co-ordinates from where the missile was fired. Then it will be easier to ascertain who fired the missile (according to the latest regional control map).
The problem is, this could be just as easily an intelligence agency deliberately firing on the aircraft from a specific region to make things in the Ukraine even worse, it's a bit like Iraq where they faked the nuclear weapons and Iran currently where they are trying to claim they have nuclear weapons and that country doesn't even really have a proper air force.