The five matchboxes

After the police receive a warning of his impending murder, stockbroker Granville Collins is found shot dead in his office. The windows were closed, the door locked. The building was under police observation: no one except Collins entered, and no one left. And upon Collins' desk lay five empty matchboxes. From this curious evidence Chief Inspector Garth of Scotland Yard is led not only to the cause of the murder, but also to a crime the Yard had labelled "Case Uncompleted."