The strict measures taken to keep his imprisonment secret resulted in a long-lasting legend about his identity. When he died there on 19 November , his inhumation certificate bore the pseudonym of "Marchioly", leading several historians to conclude the prisoner was Italian diplomat Ercole Antonio Mattioli. His true identity remains a mystery, even though it has been extensively debated by historians, and various theories have been expounded in numerous books, articles, poems, plays, and films.
During his lifetime, it was rumoured that he was a Marshal of France or a President of Parlement ; the Duke of Beaufort , or a son of Oliver Cromwell , and some of these rumours were initiated by Saint-Mars himself. Other writers similarly believed he was the King's twin or younger brother. In all, more than 50 candidates, real and hypothetical, have been proposed by historians and other authors aiming to solve the mystery.
What little is known about the prisoner is based on contemporary documents uncovered during the 19th century, mainly some of the correspondence between Saint-Mars and his superiors in Paris, initially Louvois , Louis XIV's secretary of state for war.
These documents show that the prisoner was labelled "only a valet" and that he was jailed for "what he was employed to do" before his arrest. Legend has it that no one ever saw his face, as it was hidden by a mask of black velvet cloth, later misreported by Voltaire as an iron mask.
Official documents reveal, however, that the prisoner was made to cover his face only when travelling between prisons after , or when going to prayers within the Bastille in the final years of his incarceration; modern historians believe the latter measure was imposed by Saint-Mars solely to increase his own prestige, thus causing persistent rumours to circulate about this seemingly important prisoner. In , French historian Maurice Duvivier proposed that the prisoner was Eustache Dauger de Cavoye , a nobleman associated with several political scandals of the late 17th century.
