Edgar Wallace 
Three Oak Mystery [PDF ebook] 

While brothers Lexington and Socrates Smith, authority on fingerprints and blood stains, are guests of Peter Mandle and his stepdaughter, they observe a light flashing from the direction of Mr Jethroe’s house. COME THREE OAKS, it spells in Morse. A ghostly figure is seen hurrying across the moonlit lawn. Early next morning the brothers take a stroll, and there, tied to an oak branch, is a body – a purple mark where the bullet struck.