The Fifth Amendment bars forcing witnesses to incriminate themselves, but prosecutors can compel testimony by granting immunity from prosecution. Two types exist: "transactional" immunity prevents any prosecution for the offense discussed, while "use" immunity only bars using the compelled testimony or evidence derived from it. In Kastigar v. United States (1972), the Supreme Court ruled immunity must provide "coextensive" protection to the witness. Oliver North's Iran-Contra case shows the stakes. Congress granted North limited immunity in 1987 to testify about illegal arms sales. Prosecutors later charged North with crimes he'd discussed in that immunized testimony, and a jury convicted him in 1989. But appeals courts vacated North's convictions in 1990, finding trial witnesses may have been influenced by his televised congressional testimony. In 1991, Judge Gesell dismissed all charges after key witness Robert McFarlane admitted North's immunized testimony influenced him. To prosecute immunized witnesses, the government must prove by a preponderance of evidence that its case stems from sources wholly independent of compelled testimony.