One spring morning two years ago, Karen McDonough was having tea at her dining room table. She lives in a cozy little two-bedroom house in Quincy, Massachusetts. She looked out her window and saw something unusual. "There were like 20 cars, and they all came at the same time and they parked in front of my house, across the street, up the street," McDonough said. "I just had this feeling like something really bad had happened ... like maybe somebody in the neighborhood died." Something bad was de...