Have you been watching the Netflix series, "Making a Murderer"?
A letter has surfaced that was written a few months back in September 2015 and it's a correspondence from Ken Kratz to Steven Avery. Since the context of the letter is missing, it's difficult to put the pieces together. However, it seems like they had been sending letters to each other back-and-forth.
There has been speculation from a few sources that Kratz may have wanted a confession so he can write a book about it. Hm!