No murder weapon
One of the leading theories for Samuel's death is that Sandra struck a fatal blow to his head, and an Imagine Spot shows her using a tool to do it. The idea of which tool she would have used to kill him and matching it to the wound on his head never appears as evidence in the many courtroom scenes.- This is largely deliberate, as the ambiguity over Sandra's guilt is largely the point of the movie; providing an actual murder weapon would undermine this. However, assuming that Samuel's death was murder, the first thing Sandra would do after bashing her husband over the head with a heavy tool would be to find some way of disposing of the murder weapon. The police were presumably unable to locate the actual weapon but had sufficient other evidence to bring charges.
