However, Matthiole died in 1694, which was a few years too early for him to be the masked prisoner. So, the most plausible theory points to Eustache Dauger as the prisoner. Dauger was arrested under mysterious circumstances in 1669.
Along with his warrant, there was a letter from a royal minister instructing guards to limit his contact with other prisoners. It also said to threaten him with death if he spoke about anything other than his actual needs.
Dauger was constantly relocated to several prisons, and one time, he was even transported in a covered chair so no one could see his face. All that secrecy suggested that he knew some sensitive information.
To this day, historians still don’t know who Dauger really was. He has been thought to be a lowly valet, a hidden royal, and even the twin brother of Louis XIV, a hypothesis that was popularized by Alexander Dumas.
