After the cut-scene where the suspect disappears, find the door out of the church (if you haven't moved after the cut-scene where you discover he's disappeared into the catacombs, the door will be a little bit in front of you on your right). Walk straight across to the house. Look around all you want in the house, but there aren't any clues. Go through the door in the back of the kitchen to find a room with all sorts of gruesome things inside. When you first enter this room, there is a ladder down into the catacombs on your left. Follow this Quarter Moon Murders Case Guide for more info.
