This could refer to documents related to a grand jury empaneled by Mueller since many of the same people were involved in matters Mueller is nosing around and matters related to the House committee’s document request.
Maybe we’ll know for sure soon.
This DoJ prosecutor from outside Washington, DC has been picked by Sessions to work with the DoJ IG. This prosecutor has complete authority to impanel a grand jury and appears to have done so with a focus on criminal conduct by people within DoJ and the FBI.
Since Sessions specifically mentioned Uranium One, it is very likely that Mueller is one of the targets of the grand jury since Mueller is up to his eyeballs in it.
This outside prosecutor answers to no one; not Sessions, nor Rosenstein.
Rosenstein is likely even a target in a grand jury proceeding since he signed one of the FISA applications and other corruption.