In the novel Sad Cypress, Poirot goes to see Chief Inspector Marsden, of Scotland Yard, regarding Elinor Carlisle's case. He is described as being an "experienced, kindly looking man".
Chief Inspector Marsden tells Poirot that he is sure that Elinor Carlisle killed both Laura Welman and Mary Gerrard. He mentions that it is a pleasant feeling to be sure, as the police are not out just to get a conviction as some people may think.
When Poirot says that he would like to see Elinor, Chief Inspector Marsden agrees.