Going back through this thread I noticed several posts regarding "M" and whether or not her story was believable in the eyes of LE. I just finished "The Boy In The Box" by David Stout, and what I got from his account of "M" and the investigation that followed her story, was that there was simply nothing to corroborate what she said. Yes, "M" mentioned that a passer-by had stopped to offer help when she and her mother were preparing to dump the body, but that was a detail of the case that came to light way back in 1957. She did not offer any information that only the killer (or witness to the murder and disposal) would have known.
People that knew her family and past neighbors never once saw a little boy at that residence. Which might be believable if he were actually kept in the basement for his short life, but then when did he have these numerous medical treatments and where?
The impression that I got from the book was that investigators really did try to pursue her story, but the end result was pretty much that nothing she said could be proved or disproved. I also gathered that they really gave her the benefit of the doubt, because she didn't fit the profile of your average "crazy" person. She is quite intelligent, holds various degrees in science and has a very good job. So I don't think she was written off due to presumed mental illness right off the bat.
Of course, I wasn't there to witness all of this, but I think the book did a pretty good job of explaining things. It cleared up a lot of questions I had about her and her account of what happened to the boy.