From the U.S. Attorneys Manual:
9-11.150 - Subpoenaing Targets of the Investigation
A grand jury may properly subpoena a subject or a target of the investigation and question the target about his or her involvement in the crime under investigation. See United States v. Wong, 431 U.S. 174, 179 n. 8 (1977); United States v. Washington, 431 U.S. 181, 190 n. 6 (1977); United States v. Mandujano, 425 U.S. 564, 573-75 and 584 n. 9 (1976); United States v. Dionisio, 410 U.S. 1, 10 n. 8 (1973). However, in the context of particular cases such a subpoena may carry the appearance of unfairness. Because the potential for misunderstanding is great, before a known "target" (as defined in USAM 9-11.151) is subpoenaed to testify before the grand jury about his or her involvement in the crime under investigation, an effort should be made to secure the target's voluntary appearance. If a voluntary appearance cannot be obtained, the target should be subpoenaed only after the grand jury and the United States A ttorney or the responsible Assistant Attorney General have approved the subpoena. In determining whether to approve a subpoena for a "target," careful attention will be paid to the following considerations:
The importance to the successful conduct of the grand jury's investigation of the testimony or other information sought;
Whether the substance of the testimony or other information sought could be provided by other witnesses; and
Whether the questions the prosecutor and the grand jurors intend to ask or the other information sought would be protected by a valid claim of privilege.