In that era most criminal prosecutions were conducted by private parties, either a law enforcement officer, a lawyer hired by a crime victim or his family, or even by laymen. A layman could bring a bill of indictment to the grand jury; if the grand jury found there was sufficient evidence for a trial, that the act was a crime under law, and that the court had jurisdiction, it would return the indictment to the complainant. The grand jury would then appoint the complaining party to exercise the authority of an attorney general, that is, one having a
to represent the state in the case.
The grand jury served to screen out incompetent or malicious prosecutions.The advent of official public prosecutors in the later decades of the 19th century largely displaced private prosecutions." (Wikipedia)
The Grand Jury process is purely the Prosecutor’s show. The “accused” does not have the right to testify, the policeman’s testimony in the Gardner hearing is a rare occurrence. The prosecutor can get the desire result of an indictment by being selective about the material and witnesses the Jury hears, or he can perhaps intentionally confuse the issue by presenting an overwhelming witnesses and documentation.
In Missouri the Grand Jury heard a great deal of conflicting evidence that probably could not find convincing cause to indict the policeman. The do not find an accused guilty but only that there is a great possibility that there has been a crime and the Petite Jury will make the final verdict.
We must remember that when a Grand Jury does not return the result we want it is not a miscarriage of justice but a conclusion that a body of individuals reaches from all that they were given.
In the cases where a policeman is involved there can be a fault in the fact that the local prosecutor always has a close relationship with the police. That may be cause to appoint “special prosecutors” in those cases.
(Disclaimer: I am not saying that I agree or disagree with the findings of the juries in these two cases. However especially in the Gardner case a Federal investigation if his civil rights were violated seems justified. By the tapes he did not receive standard emergency care when he stopped breathing.)