What Is a Grand Jury Investigation and How Does It Work?

A grand jury investigation is a closed-door proceeding in which a group of ordinary citizens reviews evidence presented by a federal prosecutor and decides whether there is enough reason to formally charge someone with a crime. It is not a trial. The jurors do not decide guilt. They decide only whether the evidence clears a low bar called probable cause, which is what the Fifth Amendment requires before the federal government can prosecute anyone for a serious offense.¹

Why Grand Juries Exist

The Fifth Amendment says no person can be “held to answer for a capital, or otherwise infamous crime, unless on a presentment or indictment of a Grand Jury.”¹ In practice, “infamous crime” means felony. The framers wanted a check on government power: prosecutors cannot drag someone into a federal felony trial on their word alone. A group of citizens has to agree there is enough evidence first.

This requirement binds federal prosecutions. It does not bind the states. The Supreme Court held in Hurtado v. California (1884) that states are free to charge felonies through other mechanisms, and about half of them do, using a preliminary hearing before a judge instead. If you are facing a federal matter, a grand jury is virtually guaranteed to be involved. If you are facing a state matter, whether a grand jury hears your case depends on where you live.

Who Serves on a Federal Grand Jury

A federal grand jury has 16 to 23 citizens drawn from the judicial district where the investigation is based.² They serve for a set term of up to 18 months, and the court can grant a single six-month extension if the investigation needs it. Complex white-collar and organized crime cases often run into that extension. Unlike trial jurors, who hear one case and go home, grand jurors may consider evidence on dozens of separate matters during their service.

The court appoints a foreperson and a deputy foreperson from among the jurors. The foreperson swears in witnesses, signs any indictments the jury returns, and records how many jurors voted in favor.² The vote tally itself stays sealed unless a judge orders otherwise.

Who Is Allowed in the Room

Only four categories of people may be present while the grand jury is in session: the government’s attorneys, the witness currently being questioned, a court reporter, and an interpreter if one is needed.² No judge sits in. No defense attorney is allowed inside, even if the witness has one waiting in the hallway. This is where the grand jury differs most sharply from a trial. There is no referee, and there is no opposing side.

Because no judge is present, no one rules on objections or polices the prosecutor’s questions in real time. The prosecutor runs the proceeding, selecting witnesses, choosing what evidence to put in, and framing every question. A witness can pause to step outside and consult with their attorney, but the attorney never addresses the jurors directly.³

When the jurors deliberate and vote, the room empties. Even the prosecutor leaves. Only the jurors, and an interpreter assisting a hearing-impaired or speech-impaired juror, may be present during deliberations.²

Why the Proceeding Is Secret

Grand jury proceedings are secret, and Federal Rule of Criminal Procedure 6(e) makes that secrecy enforceable. Prosecutors, jurors, interpreters, court reporters, and anyone else who handles the transcripts are prohibited from disclosing what happens inside. A knowing violation can be punished as contempt of court.²

The secrecy serves several practical purposes. It keeps targets from fleeing or destroying evidence before an indictment comes down. It protects people who are investigated but never charged from being publicly connected to a criminal probe. And it encourages honest testimony, because witnesses speak more freely when they know their words will not be broadcast to the person they are talking about.

One point catches people off guard. Witnesses themselves are generally not bound by the secrecy rules. The restrictions in Rule 6(e) cover prosecutors, jurors, and court personnel. A witness who testifies can usually leave the courthouse and tell anyone what questions they were asked, though in unusual circumstances a court may issue a specific order restricting a particular witness.

What Evidence the Grand Jury Can Hear

The formal rules of evidence that govern trials do not apply inside a grand jury room.⁴ This matters more than it sounds. At trial, hearsay — secondhand testimony about what someone else said — is generally barred. In front of a grand jury, a prosecutor can present hearsay freely. The Supreme Court confirmed in Costello v. United States that an indictment can stand even if the grand jury heard nothing but hearsay.⁵

In practice, a single FBI agent can summarize months of investigation for the grand jury without the prosecutor having to call every underlying witness or authenticate every document the way a trial would require. The grand jury can also consider evidence that might later be excluded at trial on constitutional grounds. This is one of the main reasons the process tilts so heavily toward indictment: the prosecution faces far fewer obstacles here than it will once a trial begins.

How a Grand Jury Compels Cooperation

The grand jury has sweeping authority to force people to participate. Its two main tools are subpoenas. A subpoena to testify forces a person to appear and answer questions under oath. A subpoena for documents forces a person or organization to hand over records like financial statements, emails, or corporate files. Ignoring either can lead to a civil contempt finding, which means fines or jail time until the recipient complies.

A recipient who thinks a subpoena is unreasonable can file a motion to quash it. The bar is high. The Supreme Court held in United States v. R. Enterprises, Inc. that a grand jury subpoena should be quashed only when there is “no reasonable possibility” that the requested materials will produce information relevant to the investigation. The burden sits on the person challenging the subpoena, not on the government to justify it.

Immunity and the Fifth Amendment

If a witness invokes the Fifth Amendment right against self-incrimination and refuses to answer, the government can override that refusal by obtaining a court order granting immunity. Federal immunity is “use immunity,” meaning the compelled testimony, and any evidence derived from it, cannot be used against that witness in a later prosecution.⁶ The witness can still be prosecuted later, but only if the government can show its evidence came from completely independent sources.

Immunity does not protect a witness who lies. False testimony given under an immunity order can be used in a perjury prosecution.⁶ Prosecutors often use immunity to work up the chain, compelling testimony from lower-level participants to build cases against higher-ranking targets.

If You Are the One Under Investigation

The first thing to understand after receiving a subpoena is which category you fall into. The Department of Justice distinguishes between ordinary witnesses, “subjects” (people whose conduct is within the scope of the investigation), and “targets” (people the prosecutor has substantial evidence linking to a crime). Your category affects what warnings you receive and how the process plays out.

Target Letters

When someone is a target of a federal grand jury investigation, DOJ policy calls for notifying them, usually through a “target letter.” The letter states that the recipient is a target, describes the general subject of the investigation, and outlines the recipient’s rights, including the right to refuse to answer self-incriminating questions and the right to consult an attorney. It also warns that destroying or altering related documents is a serious federal crime.⁷

A target letter is one of the clearest signs an indictment may be coming. If you receive one, hiring a criminal defense attorney immediately is urgent. The window between the letter and a possible indictment is often the best opportunity to influence what happens next.

Advice of Rights

Before any witness testifies, the prosecutor is required under DOJ policy to read an “advice of rights” on the record. Every witness is told they may refuse to answer any question whose truthful answer would tend to incriminate them, that anything they say may be used against them, and that they may step outside the room to consult counsel. If the witness is a target, the prosecutor adds a supplemental warning making the witness’s status explicit.³

Targets and subjects can ask to testify on their own behalf. The prosecutor is not legally required to allow it, but DOJ policy says such requests should “ordinarily be given favorable consideration” as long as the witness waives the privilege against self-incrimination on the record and is represented by counsel or knowingly appears without one.³ Whether to actually testify is a strategic decision that deserves careful discussion with a lawyer. You will be walking into a room where the prosecutor controls the questioning and your attorney cannot be at your side to object.

The Vote: Indictment or No Bill

After the prosecutor finishes presenting evidence, the grand jury votes. The standard is probable cause, meaning enough evidence to lead a reasonable person to believe a federal crime was committed and that the person under investigation committed it.⁸ That is a far lower bar than the “beyond a reasonable doubt” standard a trial jury must apply.

At least 12 jurors must agree to return an indictment, which is called a “true bill.”² The foreperson signs it and returns it to a magistrate judge in open court. Once filed, the indictment formally launches the criminal prosecution and typically leads to an arrest warrant. In some cases a magistrate judge will order the indictment sealed until the defendant is arrested or released on bail, which is common where there is a flight risk or where multiple targets are still being investigated.

If fewer than 12 jurors vote to indict, the result is called a “no bill,” and the foreperson reports the lack of concurrence to the magistrate judge in writing.² A no bill stops that particular proceeding, but it does not permanently end the matter. A prosecutor can re-present the case to a different grand jury, potentially with new evidence or a different framing. No double jeopardy protection attaches, because the grand jury stage is not a trial and there has been no acquittal. The main practical limit is the statute of limitations for the underlying offense.

In federal court, that debate is largely academic. Federal grand juries indict in the overwhelming majority of cases, well above 99 percent by most available data. The combination of a low probable cause standard, relaxed evidence rules, no defense participation, and prosecutorial control over what jurors see makes a no bill genuinely rare. The real screening happens before and after: when prosecutors decide which cases to bring in the first place, and later, when a trial jury applies the much tougher beyond-a-reasonable-doubt standard.

1
Legal Information Institute. U.S. Constitution – Fifth Amendment
2
Legal Information Institute. Federal Rules of Criminal Procedure Rule 6 – The Grand Jury
3
United States Department of Justice. Justice Manual 9-11.000 – Grand Jury
4
United States Courts. Federal Rules of Evidence
5
Justia. Costello v. United States, 350 U.S. 359 (1956)
6
Office of the Law Revision Counsel. 18 U.S. Code 6002 – Immunity Generally
7
United States Department of Justice. Criminal Resource Manual 160 – Sample Target Letter
8
United States Courts. Handbook for Federal Grand Jurors