They whittle down the suspects to Mark and Damien—of course, Rob thinks Mark did it, for no reason whatsoever.
Sam mentions that Damien is the only left-handed one in the group, so they call him in first.
They think it's fishy when Damien insists he's never seen Katy before, because she often hung around the site.
Meanwhile, Sam is getting nowhere interrogating Mark.
Rob decides to quiz Damien about Mark. Did he ever hit anyone? Isn't he just the worst? He's not really that cute is he?
They break for food and feed Damien some pizza.
Sophie calls Rob to tell him that the trowel was definitely the rape weapon; also, they found Katy's Hello Kitty flashlight under another tarp. Importantly, though, they didn't find any traces of blood in Mark's car.
Back in the interrogation room, they ask Damien about his alibi: He was home with mommy.
The catch is that mommy was asleep, and Damien could have snuck out of the house and gone… in the woods.
They turn the screws on Damien, showing him pictures of Katy, dead and alive, and telling him that they don't think the killer meant to do it—he was a nice guy, a victim of circumstance.
Finally, Damian confesses: "It just happened" (20.136).