Starfleet Investigative Services was an agency within Starfleet in the 24th century. The purpose of this agency was to investigate crimes by Starfleet personnel.
Prior to his death in 2357 aboard the USS Pegasus, Ensign Phil Wallace was cleared of wrongdoing by this agency. (ENT: "These Are the Voyages...")
This agency was mentioned in the text which accompanied the personnel files of Pegasus crewmembers seen in the episode.