What made the detectives suspect that the murder involves some specific group of people in Vermissa Valley?

Still more clues begin to present themselves in terms of Douglas’s past and the sense that he was under some sort of dark cloud. It is clear he came to England to escape some sort of persecution in America, and it is connected with the shadowy “Valley of Fear.” This ominous appellation is a physical place as well as a symbolic one in the sense that Douglas is psychologically stuck in that infernal valley.