Witness immunity

Witness immunity from prosecution occurs when a prosecutor grants immunity to a witness in exchange for testimony or production of other evidence. It is immunity because the prosecutor essentially agrees never to prosecute the crime that the witness might have committed in exchange for the said evidence.

In the United States, the prosecution may grant immunity in one of two forms. Transactional immunity, colloquially known as "blanket" or "total" immunity, completely protects the witness from future prosecution for crimes related to his or her testimony. Use and derivative use immunity prevents the prosecution only from using the witness's own testimony or any evidence derived from the testimony against the witness. However, if the prosecutor acquire evidence substantiating the supposed crime, independently of the witness's testimony, the witness may then be prosecuted for the crime. Prosecutors at the state level may offer a witness either transactional or use and derivative use immunity, but at the federal level, use and derivative use immunity is quite common.

Witnesses compelled by subpoena to appear before a grand jury are entitled to receive immunity in exchange for their testimony. The grant of immunity impairs the witness's right to invoke the Fifth Amendment protection against self-incrimination as a legal basis for refusing to testify. Per 18 U.S.C. § 6002, a witness who has been granted immunity but refuses to offer testimony may be held in contempt of the court that issued the subpoena. In addition, grand jury witnesses may be prosecuted for perjury or making false statements in their testimony.

In Kastigar v. United States, 406 U.S. 441 (1972), the US Supreme Court confronted the issue of the type of immunity, use or transactional, constitutionally required to compel testimony. The Court ruled that the grant of use and derivative use immunity is sufficient.

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