The pain inflicted by the Boston Marathon bombing was one of the few aspects of that act of terror that were immediately evident and certain. Such is the nature of the crime that hundreds of individuals and families will suffer loss and trauma for many years to come. This book, however, is not about that pain. It is about something that, whatever evidence is unearthed, will never be entirely certain: it is about the tragedy that preceded the bombing, the reasons that led to it, and its invisible victims.