How cops get the proper suspect ...

Police actions (or perhaps more properly inactions):

Focused immediately on the missing woman's husband who stated he was on a walkway at the time and passed a surveillance camera. The police waited over six months and then reported that the film for that day had been either lost or recorded over. The only real problem is that the surveillance camera was on the exterior of their own police station and could have been obtained by them in two minutes flat. The police also seized his shoes and after cleaning them, pronounced them to be insufficiently muddy for him to have taken the path that he claimed to have taken.

Conducted three forensic examinations of a motor vehicle but despite it being a major case with over forty officers assigned to it, they claim they never photographed the process and thus the unprecedented fourth forensic test just happened to find an inculpatory blood stain in a highly suggestive location. Of course the reagent used was luminol which is only a presumptive field test never to be relied upon without further serology work at the lab but the police felt there was no need for more detailed serology work. Anyone remember the Dingo Baby case from Australia wherein Luminol reacted with the underlying rust proofing in the automobile and was declared to be indicative of arterial spray?

Discovered the missing woman had withdrawn large sums from a number of banks then visited a man who knew of the withdrawals and shortly thereafter the woman went missing but the police never viewed him as a suspect or investigated his whereabouts even though what little he did tell police was proven to be lies. Despite his attempts to thoroughly clean his car later examination did find traces of blood. Witnesses who came forward to report the boyfriends car in close proximity to the victim's car were ignored. Witnesses who reported seeing the victim alive the day after the husband was supposed to have killed her were ignored despite one of them having known the victim for ten years.

Is it any wonder how the cops get the proper suspect? It is because they are the ones who choose who should be viewed as a suspect.