Mystic River is set in a Boston community which is rocked by the fatal shooting of a young woman. Everyone begins to suspect Dave, who was seen with the victim before the murder and then came home later with an injured hand and blood on his clothes, supposedly from a mugging.
As the cops investigate the murder, several days go by. Eventually the young woman's grief-stricken father confronts Dave, who then says he got the injury and blood from killing a child molester who he saw with an under-age prostitute. The story seems unbelievable, and the grief-stricken father kills Dave.
Of course, the very next day, the cops announce that they caught someone else who confessed to killing the young woman -- and by the way, in an unrelated case, a child molester had been killed that same night, and the cops suspect Dave did it.
Why hadn't anyone in the entire movie -- particularly the cops -- mentioned that there had been a second murder (the child molester) on the same night as the young woman was killed, presumably in the same vicinity where Dave had been?