The Thames Crossbow Murders - Vance And Shepherd Mysteries, #3

After a crossbow bolt kills a birdwatcher in a remote site on the Thames, the Metropolitan Police discover they have another serial killer on their hands. The killer leaves no trace, and Detective Inspectors Shepherd and Vance are faced with a puzzle that threatens to erupt into an international scandal. Soon after, a virtuoso pianist loses his life in the same manner. The murders are all connected to the river Thames, but the waters are muddied by the involvement of British and Russian secret services.
After a man is arrested for the murder, Shepherd is certain that he is the culprit. But what do the international organizations have to do with the case, and who is the killer?