What made Mason think that Mr Douglas was assassinated and it was not a suicide?

Upon inspection of the dead man, it is revealed that he has been killed with a sawed off shotgun, an American weapon. Beside the disfigured body lies a card inscribed 'V.V. 341'. Other clues in the room include a bloody footprint on the windowsill, a missing dumb-bell, and the fact that the man’s wedding ring is missing. Another clue discovered soon after is a bicycle, concealed just outside the house grounds. Interrogation of the house servants reveals little, and after Mrs. Douglas, wife of the murdered man, and Cecil Barker, a friend of the family, are questioned, Holmes reveals to Watson that he believes the two are in a conspiracy together.