I've worked with Germans in the past and one thing I will say about them is they are very efficient and very precise.
So if they have written to K&GM to tell them Maddie is dead and they know how she died, then you can be sure they have solid irrefutable evidence.
Since CB isn't talking yet, there can only be a limited number of possibilities that can deliver that level of certainty.
1) Video
2) Photographs
3) Physical objects/evidence that can lead only to the conclusion that a person is dead (if you catch my drift)
4) Some written confession the suspect produced at some point in the past (email, text, diary, etc)
Someone saying "he said this, I saw that" wouldn't cut it to write to the parents with such finality, since it would only be hearsay and nothing like the "concrete evidence" they are claiming to have.