No indictment in Ferguson case

[Travelman]

96. It varies a little bit from state to state

A grand jury is specifically there to determine not innocence or guilt, but to determine whether or not it is likely that a crime occurred. They are charged with determining whether a preponderance of the evidence suggests that a crime occurred, and if so, what that crime would be (according to the statute). Technically, a grand jury does not have to indict a particular person; leaving aside the details of this case, it is possible (though very rare) that a grand jury can return a report that because John Smith was shot in the back of the head, it's almost certain that a crime was committed, but they don't find a preponderance of the evidence that Joe Blow is the one who committed that crime.

Not all cases involve a grand jury. A prosecutor may take the evidence, testimony, etc. that they have to a judge, and that judge may make a determination of probable cause. In some jurisdictions, a grand jury is guaranteed for any criminal indictment, even for minor things like traffic tickets, if the defendant demands it. In some jurisdictions, a grand jury is required by statute for certain crimes (most typically the top murder charges).

Each state sets up their own grand jury rules on their own. In the case of Missouri, the grand jury is selected from a standard jury pool, serves for some set length of time, and has twelve members. At least nine are required to agree in order to return an indictment. Other places are different: some require a simple majority, some require a unanimous grand jury, others require something in between. Some grand juries are much larger, 18-25 grand jurors, some are smaller, just six or eight.

Grand juries are often political cover for prosecutors in high-profile cases. If it's going to be problematic whether charges are brought or not, a prosecutor can toss it to the grand jury, and then no matter what the decision, when anyone asks, they can just say "well, this is the indictment that the grand jury returned." But that is far from the ONLY reason grand juries are convened. Sometimes, it's just a toss-up from the prosecutor's perspective, and they want the grand jury to make that coin-toss. Sometimes, a prosecutor wants to go through the grand jury frequently in order to preserve an appearance of impartiality. And, as noted above, sometimes it's mandated by statute.

Wilson was not tried here because that's not the function of a grand jury. A grand jury is the "gateway" to someone being prosecuted. It's a check upon prosecutors in general. Had the grand jury returned an indictment, then that would have triggered an arrest, arraignment (formal reading of the charges in open court), and then a trial (assuming that there was no plea agreement or the charges tossed out for some reason or another or something else to disrupt the trial).