Plaintiff’s suit against his investigation is barred by Younger abstention, too. It doesn’t require a pending case. Cook v. Harris County, 2019 U.S. Dist. LEXIS 87297 (S.D. Tex. May 23, 2019):
The first question is whether the criminal investigation against Mr. Woodfill can be considered an “ongoing state proceeding,” when he has not yet been charged. The quintessential Younger abstention scenario involves a request for a federal court to enjoin a state prosecution of the federal plaintiff.
The Fifth Circuit has held that events prior to the formal commencement of criminal proceedings (a charge or indictment) may qualify as ongoing state proceedings. For example, in Texas Association of Business, the Fifth Circuit found that state grand jury proceedings qualified as ongoing state proceedings. 388 F.3d at 519-21. In so doing, the court emphasized that while Younger applies to some administrative and civil proceedings, “[i]t is the criminal law arena where the federal courts’ deference to state courts has been most pronounced.” Id. at 520. The court noted that the functions of grand jury proceedings “pertain directly to the enforcement of the state’s criminal laws.” Id.