A grand jury hands down indictments that kick off a criminal trial.  From findlaw.com:

A grand jury helps the prosecutor decide whether to file criminal charges against a suspect. While the grand jury system may vary by jurisdiction, it is most often used in federal and state felony cases. Federal grand juries typically consist of 16 to 23 people. Grand jury members may have jury duty for months at a time. Sometimes, the district attorney will choose to proceed with a preliminary hearing rather than a grand jury. Both determine whether there is enough evidence to indict someone.

A grand jury is private [i.e. secret, as opposed to a public preliminary hearing]. Besides the jurors, only the prosecutor may attend to present evidence to the jurors. The prosecutor decides what the grand jury investigates, the evidence to present, and what documents to subpoena. A grand jury does not have to be unanimous to issue an indictment. Depending on the jurisdiction, two-thirds or three-quarters of the individual grand jurors must agree. Prosecutors use grand jury proceedings as test runs for trials. But if the prosecutor strongly disagrees with a grand jury, they may ignore the decision.

The article also states that “grand juries will work closely with the prosecutor, who will explain the law to the jurors.  The jurors then have the power to view almost any evidence they wish and question anyone they like.”

Since prosecutors decide what evidence gets investigated, subpoenaed, and disclosed to the jurors, are they then required to disclose exculpatory evidence?  It’s not a trial, after all — the prosecutor’s job is to present evidence to convince the jurors of guilt.

In some states and in federal court, they don’t have to present any evidence suggesting that the target is innocent. The rationale is that the defendant will have the opportunity to present this evidence at trial. ... In most states, prosecutors can’t present half-truths to grand juries. If prosecutors have strong, credible evidence that points to innocence, they must divulge it. That doesn’t mean, however, that they have to offer every piece of evidence that’s helpful to the accused or that might be used at trial by the defense.

In practice, however, the enforcement of this “must divulge” rule is left to the prosecutor’s conscience.

Can a witness who is subpoenaed before a grand jury invoke the 5th Amendment? That’s up to the prosecutor, but

if the prosecution and the grand jury insist on the appearance, the Fifth Amendment privilege must be asserted on a question-by-question basis. ... “[B]efore a claim of privilege can be sustained, the witness should be put under oath and the party calling him be permitted to begin his interrogation. Then, the witness may invoke his privilege with regard to the specific question and the court is in a position to make the decision as to whether the answer might tend to incriminate the witness.”  People v. Harris (1979) 93 Cal.App.3d 103, 117; see Celaya, 855 F.2d at 863.

During Robert Mueller’s special prosecution investigation of President Trump, a grand jury was used to compel witnesses to turn over documents as part of the evidence-gathering.  A separate grand jury was formed just to investigate General Michael Flynn.

It’s clear that prosecutors have a good deal of discretionary power to direct the outcome of a grand jury proceeding, and the defense is left in the dark.  Proponents of grand juries argue that the legally mandated secrecy protects the reputation of the innocently accused, but that secrecy is what makes grand juries attractive to prosecutors.  It should come as no surprise that grand juries almost always return indictments:

According to a U.S. Department of Justice study, “Grand juries are notorious for being ‘rubberstamps’ for the prosecutor for virtually all routine criminal matters.” (McDonald, William F., Plea Bargaining: Critical Issues and Common Practices [1985].) It’s also suggested that grand juries rubber stamp prosecutors’ charges because grand jurors are not adept at evaluating evidence like judges are — making it easier to convince a grand jury than a judge that the defendant should stand for trial.

Is it not reasonable to presume that district attorneys and assistant prosecutors are well educated and trained in criminal law, especially the laws of evidence?  They ought to be well qualified to decide if a case should go to trial or not.  Can the driving force behind convening a grand jury sometimes be to cover the keister of a politically ambitious D.A.?  If he were to blow a high-profile criminal trial, it could then be said, “Hey, it not my fault, we vetted the case through a grand jury!”

Prosecutors have plenty of laws, statutes, and tactics at their disposal.  They should not need the lopsided power of grand juries to do their jobs effectively.

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