Despite the tireless work of crime investigators, and the wealth of modern forensic techniques available to them, a disturbing number of crimes remain unsolved. This may be because the perpetrator cannot be identified -- or, if identified, cannot be traced -- or because the evidence is insufficient to bring the case to trial. A major proportion of these cases, unfortunately, are of murder, or suspected murder, and this book takes a look at six of the most challenging cases that have frustrated investigators in recent years