The grand jury consists of twenty-three jurors, who are citizens in the county they are called to serve in. They are asked to serve for a period of time, usually a month. The number of times they meet per month varies by the size of the county. In New York City, the grand juries will meet twenty times during their term. In small counties, they might meet four times.
In simple terms, every felony must be presented to a grand jury in New York State, unless the defendant pleads guilty prior to the presentation. The grand jury has the power to indict a person (formally accuse the person of a crime) or file a no bill (dismiss the charges). Twelve of the grand jurors must vote to indict or twelve must vote to dismiss. If you do the math, a majority is needed to do either action.
The grand jury may vote to indict a person if there is reasonable cause to believe the person committed an offense. That is significantly less than the standard at trial a jury must follow which allows the jury to convict only if they find the person guilty beyond a reasonable doubt. Due to this, many cases will pass the grand jury stage, but fail at trial.
There are really two types of cases a prosecutor presents to the grand jury. Most often we present cases where the police have investigated a crime and arrested a suspect (We'll call these "charged crimes"). Probably once a year in a large jurisdiction, each prosecutor presents a case without charges filed and uses the grand jury as the investigating body (We'll call these "uncharged crimes").
The difference? The charged crimes are usually fully investigated or close to it. The grand jury is a safeguard to decide if there is enough evidence to proceed to trial. The uncharged crimes are usually not fully investigated and require the power that only a grand jury can provide. A prosecutor usually has an idea that a crime occurred and a possible suspect at this point, but is never sure of where the proof will actually lead.
A grand jury can issue subpoenas, demanding to hear testimony or see documents. The police can merely request. If a person refuses to talk to the police or the prosecutors or provide requested evidence, a grand jury is how the prosecutor will force the person to testify or turn over the evidence. Many times I have had witnesses refuse to talk to anyone, but I obtained information they were an eyewitness or an earwitness (heard a confession). This required me to present the case to a grand jury and force the potential witness to testify. Granted, the grand jury is the last step in the investigative process because once the grand jury issues subpoenas the secret investigation no longer exists. We try to do everything we can without using the grand jury's investigative power, but sometimes cases require it.
Everyone is sworn to secrecy concerning the grand jury proceedings, except for the testifying witness. This is meant to protect the witness' privacy and alleviate pressure. Once the testimony is presented, the prosecutor then charges a grand jury by reading the applicable penal law violations and leaves to let the grand jury decide. The grand jury can request to consider charges the prosecutor does not submit, but the request must be based on evidence. If a grand juror wants to consider murder in the first degree even if the defendant is sixteen, the prosecutor cannot submit the charges because an element of murder first is the defendant be nineteen or over.
The issue arises as to a prosecutor's ethical obligations. If the grand jury investigation does not yield the expected evidence should the case be prosecuted further? To put it another way, if the prosecutor feels they cannot prove the case beyond a reasonable doubt at trial but is confident there is enough evidence to satisfy the lower grand jury standard, should they indict it or ask to file a no bill? Should the victim get their day in court in an unproveable case? Isn't that how innocent people get convicted because the prosecutor just takes a chance?
My office has a policy that we do not indict a case unless we can prove it beyond a reasonable doubt at trial. This is to guard against wrongful convictions. My District Attorney gets a lot of negative publicity for this stance because they claim he only takes the "winners" and "slam dunks." Even with his policy, we still lose some cases after trial. There are no winners or slam dunks before a jury. If it is a close call as to the proof at trial, we will let the grand jury decide and then move the case to trial.
The grand jury is a screening tool for the prosecutor just as much as it is a procedural safeguard for the defendant.