somebody knew what they were doing
This film is particularly good if you've ever done any reading on the subject of false confessions and other aspects of police interrogations, such as fixing on a single theory and then getting tunnel vision re the evidence; making one critical prejudgment and then shaping every subsequent decision to confirm that first decision; lying during interrogations, to make the suspect (or interviewee) believe the case against him is much stronger than it is; and so on. Whoever advised on this film _really_ knew what he/she was doing.
(I'm exempting certain obvious flaws such as the fact that nobody seems particularly interested in going out to where the burnt lean-to is supposed to be, look for charred remains, etc.)
Also on target is what Fleming said about the tendency of police to let public pressure over unsolved crimes get in the way of investigative thoroughness and objectivity. That is absolutely true--and further evidence that it occurred here comes in the fact that they keep repeating the fact that there is no link between the disappearances and not even any bodies, if I recall correctly. That's the big fact that those convinced of Fleming's guilt are missing--it's nowhere near certain that a crime has even been committed at all. The whole film seems to be more about our need to gather up unexplained occurrences and put a structure or meaning to them.
It's also true that the occasional interviewee under threat or pressure will make a false confession that is deliberately intended to mislead police, who will then be confounded and/or embarrassed by the discovery that they've been duped (or so the suspect thinks). Not a good idea, but I think most people would be surprised what they would do when under that kind of duress, when told people had identified them at the scene, that fingerprints matched, etc.