Prosecutors don't investigate. Witnesses are first questioned by FBI or other investigatory bodies. Only if the investigation turns up probable cause will a prosecutor present that evidence to a grand jury and go from there. The witnesses don't necessarily appear before the GJ, just the evidence.
That said, we don't know what's going on, other than the usual, people who either don't understand the system are speculating their asses off, and ignorant people are wasting time trying to analyze speculation.,