must be bullet-proof alias backed by hard facts and evidence
As prosecutore he can talk about evidence, but hard facts is simple - plane hitted by strike elements in cabin from forward and above to below.
Earth (even Germany) have not much sources which can do it. Missile surface-air can do it. Missile air-air can but read below. Aircannon - cannot (even if evidence show it or prosecutors claim they found it). MLRS shell - cannot (very popular stupid idea from russian press/patriots). Explosive on board - cannot.
Shape of holes strip off all possible air-air missiles since they use continous rods. But holes is absolutely hard fact. You can easy see that fact on multiple photos. Dont need to be main prosecutor because even he cannot change that shape.
http://i.imgur.com/2EGBamN.jpg
So hard facts say Buk missile (other missiles have other strike elements btw) down MH17. Germany reject to help Ukraine for gain control over area. Now Germany want evidence. Of course it right time, when russian specialists place all needed evidence signs on controlled place.