On Long Island, a farmer finds a duck pond turned red with blood. On the Lower East Side, two boys playing at a pier discover a floating human torso wrapped tightly in oilcloth. Blueberry pickers near Harlem stumble upon neatly severed limbs in an overgrown ditch. Clues to a horrifying crime are turning up all over New York, but the police are baffled: There are no witnesses, no motives, no suspects. The grisly finds that began on the afternoon of...