In the 1957 case "Jencks v. United States" the United States Supreme Court ruled that a defendant must have access to government witnesses who will testify against him in a criminal trial, and must also have access to any documents pertaining to that testimony. This includes papers, documents, written statements and the like. This led to passage of the Jencks Act, 18 USC, Part II, Chapter 223, § 3500, which allows for "subpoena duces tecum" of relevant government documents, but only after a government agent or employee has testified at trial. There can be no pre-trial discovery. The subpoena is allowed by the trial judge. The government has the right to deny access to the documents. This may be due to the sensitive nature of the documents, or because they are classified.