LIVINGSTON, Tenn. — Four days a week Todd Matthews earns $11.50 an hour working for an automotive parts supplier. He punches in at 4:15 a.m., punches out nearly 11 hours later, then drives half a mile to his little beige house on a hill where, in the distance, he can glimpse the Appalachian mountains. He spends the next seven to eight hours at his desk, beneath shelves lined with miniature plastic skulls, immersed in a very different world. Their faces seem to float from his computer — morgue photographs, artist sketches, forensic reconstructions — thousands of dead eyes staring from...