Grand juries are called by a federal judge and are required to supervise them. But judges almost never interfere. The grand jury is led by a federal prosecutor. He cannot testify or be present while the jury deliberates. But most federal grand juries will return any indictment presented to it. This led to the famous saying "a prosecutor can get a grand jury to indict a ham sandwich." They can subpoena anything they want with very few restrictions. In United States v. R. Enterprises, Inc., 498 U.S. 292 (1991), the Supreme Court of the United States ruled "federal grand jury subpoenas are presumed to be reasonable and the burden of showing unreasonableness is on the recipient."