The first piece of evidence appears on Walter Eckert’s desk in a locked office to which he has the only key. It is wrapped in brown paper, neatly labeled with his name, no return address. He unwraps it with wary hands. Cheap plywood, as if from a construction site wall, pasted with a handbill-sized poster. It could be advertising any event around town—a rock band no one has ever heard of, an avant garde art exhibition no one will ever see—but it appears to advertise nothing at all.