Deduction.

     Based on the evidence, I deducted the identity of the passengers.


Countess Adrenyi - Helena Goldenberg - the younger daughter of Linda Arden and sister to Sonia Armstrong.


Colonel Arbuthnot - A close friend of Colonel Armstrong


Mary Debenham - Was companion governess in the Armstrong Residence at the time of the kidnapping


Antonio Foscarelli - Was the driver of the Armstrong family


Greta Ohlsson - Was the nurse in charge of Daisy Armstrong


Edward Masterman - Was Colonel Armstrong's batman in the war and his servant in NY afterwards


Cyrus Hardman - Was in love with the servant which threw herself from a window


Caroline Hubbard - Linda Arden - Famous actress and the mother of Sonia Armstrong and Helena Goldenberg


Princess Dragomiroff - A close friend of Linda Arden and the Godmother to Sonia Armstrong


Pierre Michel - Father of the nursemaid who committed suicide


Hector MacQueen - Was in partnership with the Armstrong family


Hildegarde Schmidt - Was the cook in Armstrong household


     It came out, I was perfectly right about them, they admitted it themselves.

There are two possible solutions of this case...