We see the story all the time here--the most likely person to kill a kid is the adult male who isn't the father but who is now living with the mother. And the amount of evidence that the police "lost," the witness statements that weren't recorded and which the witnesses later said police utterly mischaracterized is just amazing.
And the real killer will never be arrested because this will close the case. This is just as much a travesty of justice and the original investigation. But the guys really don’t have much of a choice.