From the USDOJ Criminal Resource Manual:
U.S. District Attorneys are encouraged to notify people when they are no longer targets:
The United States Attorney has the discretion to notify an individual, who has been the target of a grand jury investigation, that the individual is no longer considered to be a target by the United States Attorney’s Office. Such a notification should be provided only by the United States Attorney having cognizance over the grand jury investigation.
Discontinuation of target status may be appropriate when:
The target previously has been notified by the government that he or she was a target of the investigation; and,
The criminal investigation involving the target has been discontinued without an indictment being returned charging the target, or the government receives evidence in a continuing investigation that conclusively establishes that target status has ended as to this individual.
There may be other circumstances in which the United States Attorney may exercise discretion to provide such notification such as when government action has resulted in public knowledge of the investigation.
The United States Attorney may decline to issue such notification if the notification would adversely affect the integrity of the investigation or the grand jury process, or for other appropriate reasons. No explanation need be provided for declining such a request.
If the United States Attorney concludes that the notification is appropriate, the language of the notification may be tailored to the particular case. In a particular case, for example, the language of the notification may be drafted to preclude the target from using the notification as a “clean bill of health” or testimonial.
The delivering of such a notification to a target or the attorney for the target shall not preclude the United States Attorney’s Office or the grand jury having cognizance over the investigation (or any other grand jury) from reinstituting such an investigation without notification to the target, or the attorney for the target, if, in the opinion of that or any other grand jury, or any United States Attorney’s Office, circumstances warrant such a reinstitution.