Witness Immunity From Prosecution
A witness may be granted immunity from prosecution by way of federal or state law. Immunity is granted in exchange for a witness's testimony about a certain event. There are two types of immunity available, use immunity or transactional immunity.
States may adopt the theory of use immunity as the federal government has adopted. Use immunity permits the state or federal government to prosecute the witness using evidence obtained independently of the witness's immunized testimony. The state or federal government is prohibited from derivative use of immunized testimony for the prosecution of the immunized witness. If the state or federal government chooses to prosecute the witness for an offense they must prove that the evidence is based upon an independent and legitimate source. The state or federal government has an affirmative duty to prove that its evidence was derived from a legitimate source independent from the compelled testimony.
The federal government no longer uses this type of immunity, however some states still use this method. Transactional immunity is defined as immunity that protects a witness from prosecution of the offense involved. For example, if the witness testified about participating in a narcotics transaction, she could not thereafter be charged for a crime that stemmed from her immunized testimony. Whereas with use immunity, the witness could be charged with an offense stemming from her immunized testimony so long as the state or federal government was able to show that the evidence used against her was derived from an independent source and not from her immunized testimony.
Informal vs. Formal Immunity
Testimony that is given under informal immunity is not compelled testimony, but is testimony that is given pursuant to an agreement. Informal immunity is voluntary in nature. A grant of informal immunity can either prohibit the government's derivative use of the witness's testimony or not. If the grant of informal immunity provides for derivative use of the testimony, the derivative use will be upheld. States are not bound to any grant of informal immunity. The principles of contract law are applicable in determining the scope of informal immunity.
Formal immunity is synonymous with statutory immunity. Statutory immunity is provided by way of a state or federal statute. The type of immunity provided may be either use or transactional depending upon the applicable jurisdiction.