An old couple knelt side by side in an otherwise empty small - town church - their heads resting on their hands in a picture of devotion. Both had small, tiny bullet holes in the backs of their skulls. To local police their slaying looks like a one-off act of retribution. At the same time, in Minneapolis, teams of detectives are scrambling to stop a sickeningly inventive serial killer striking again in a city paralysed by fear.