My nitpick is that the prosecutor doesn't have to "reinvestigate" or "parallel investigate." Like any police/prosecutor hand off, the prosecutor relies on the investigator for most evidence.
As I mentioned in my previous, not only is OIG limited in terms of power (investigation only), he is also limited in terms of target (DOJ only, for Horowitz). And in range of target, he is limited in power to compel testimony, being restricted to government employees - once out, also out of OIG reach.
I totally agree that assignment (the word I would use in conjunction with an inside prosecutor) or appointment (the word Sessions used, and the word used in conjunction with Special Counsel) of a prosecutor is absolutely essential. Signs are that this investigator (Horowitz) is already partnered with a prosecutorial team.
Parallel? Think mirror!