One September afternoon in 2003, a woman in Tucson, Arizona, heard a knock on her front door and answered it. She was surprised to see a Tucson police detective standing on her front steps, and even more surprised when the detective started asking a bunch of questions about her next door neighbor. The woman was a law-abiding citizen, and she didn’t know anything. The detective gave her his business card and asked her to call him if that changed, but as she closed the door, she was pretty sure she would never have any reason to talk to him again. That was until a few days later, when she looked out her front window and saw two sweaty men jump out of a van and walk up the steps into her neighbor’s house. She watched for a few minutes, with her heart racing, realizing that what she was watching might very well be the coverup of a murder. She let the curtain fall and ran to her purse, to dig out the detective’s business card.