The Special Investigator for the Director was a specially appointed agent of the Office of State Security.
Special investigators where hand-picked by Oscar Saint-Just and appeared to have been his personal hatchet men when dealing with serious issues of discipline, loyalty and competence within StateSec itself.
The office appeared to have stood outside the normal StateSec rank structure and carried with it considerable authority; when Victor Cachat was appointed as an S.I. for the La Martine Sector, he was only a StateSec Captain, but the office gave him sufficient authority to effectively take over the entire La Martine Navy detachment, both regular and StateSec. During his purge of "corrupt elements" of StateSec, he took control of the entire sector, ordering arrests, interrogations, executions, and military operations on his own authority. It would appear likely that the office of special investigator was appointed on an "as needed" basis for special situations. (HHA4.5: F)