I was reading over this answer, and it occurred to me that, when offering immunity, the prosecutor does not have foreknowledge of what the defendant will ultimately reveal. (After all, if the prosecutor knew the information, there would be no need for the immunity itself.)

So, if someone is investigated for crime A, and is offered immunity for cooperation, could the defendant then also admit to some additional, only marginally-related crime? Or something altogether unrelated? In practice, are strict limits on what the immunity would entail communicated prior to any admission of guilt?

  • 1
    Sometimes it can be quite complicated and there are several kinds of immunity that can be offered.
    – ohwilleke
    Commented Dec 20, 2017 at 20:05
  • I'm voting to close this question as off-topic because it is so broad as to be effectively opinion-based. Different prosecutors have different policies and rules, as well as possibly different laws on immunity. Commented Dec 20, 2017 at 22:40
  • Disagreed, @TimLymington. The fact that various prosecutors have various policies and rules inherently means an answer is not necessarily opinion-based. The fact that various theories exist does not make something merely opinion-based.
    – A.fm.
    Commented Dec 21, 2017 at 5:57
  • Prosecutors don't offer immunity to find out what the witness knows; they offer it in order to get the witness to say it under oath on the witness stand.
    – EvilSnack
    Commented Mar 6, 2022 at 2:13

1 Answer 1


could the defendant then also admit to some additional, only marginally-related crime?

I suppose, but why would he or she?

Or something altogether unrelated?

Again, of course that could happen, but it is not clear what motivation there would be to admit to random crimes unrelated to what the person is being questioned about.

are strict limits on what the immunity would entail communicated prior to any admission of guilt?

Yep. Also, the content of what the defendant (or witness - it's not always a defendant) will say is also communicated prior to any admission.

In summary:

Prosecutors can grant immunity in exchange for cooperation, or reach a plea bargain for a reduced sentence. Before that can happen, prosecutors need to know what the witness will say and how credible that testimony is. There is nothing worse than making a deal before knowing what you are getting in return.

In order to make a deal, prosecutors and defense lawyers engage in what is known as a “proffer” session, in which the witness is interviewed and the information being offered is gauged for its usefulness. Once someone talks with the government, however, the Fifth Amendment privilege is usually lost.

To deal with that issue, the Justice Department usually enters into an informal letter immunity agreement — sometimes called “Queen for a Day” immunity after the 1960s television game show. Such immunity is only good for the particular proffer session and ends as soon as the two sides are finished.

Under the letter agreement, the witness provides complete information to the government without waiving the protections of the Fifth Amendment.

Unlike immunity granted under the federal statute 18 U.S.C. § 6002, this is a more informal arrangement that is governed by the terms of the agreement worked out by the parties. Like any contract, it is subject to negotiation, and the leverage one party has will affect how much protection is afforded if there is not a resolution.

Prosecutors typically seek to use the statement to develop new leads and introduce new evidence gathered as a result of the information provided. This is not permissible when full statutory immunity is granted to a witness.

Another provision often sought by the government allows prosecutors to introduce the statements to impeach a defendant if the case proceeds to trial and the person testifies in a way that is contradicted by the statements made in the proffer session. This is included as a way to deter a witness from giving one story when seeking to resolve a case, and then saying something different at trial to avoid a conviction.

The federal evidence rules usually do not permit the introduction of statements made during negotiations related to a potential settlement of a case. The letter immunity is intended to get around that by having the witness waive the protection so that the statements can be used at a trial.

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