Here is a link to, "A Grand Facade: How the Grand Jury Was Captured by Government."
The prosecutor calls the shots and dominates the entire grand jury process. The prosecutor decides what matters will be investigated, what subpoenas will issue, which witnesses will testify, which witnesses will receive “immunity,” and what charges will be included in each indictment.
Because defense counsel are barred from the grand jury room and because there is no judge overseeing the process, the grand jurors naturally defer to the prosecutor since he is the most knowledgeable official on the scene. That overbearing presence explains the old saw that a competent prosecutor can “get a grand jury to indict a ham sandwich” if he is really determined to do so.
And the reverse also holds true: If a prosecutor does not want an indictment, he can secure that outcome if he is really determined to do so.