Arthur Gask 
The Hangman’s Knot [EPUB ebook] 

Gilbert Larose is again called from retirement to solve a mystery—if anything, a mystery more baffling than those which have made his name in the past. Six murders in seven weeks and Scotland Yard is helpless. The crimes give no clue—not even a similarity of method to connect them. Just the whine of a bullet, the stab of a knife or the crash of a bludgeon, and the murderer has slunk back into the blackness of the night. Six times in seven weeks a coroner's jury returns the verdict “Murder by some person or persons unknown.”