What is a 'Unindicted Co-Conspirator' and why can't you identify them by name?

[SummerSnow]

9-11.130 - Limitation on Naming Persons as Unindicted Co-Conspirators
In the absence of some significant justification, federal prosecutors generally should not identify unindicted co-conspirators in conspiracy indictments. The practice of naming individuals as unindicted co-conspirators in an indictment charging a criminal conspiracy has been severely criticized in United States v. Briggs, 514 F.2d 794 (5th Cir. 1975).

Ordinarily, there is no need to name a person as an unindicted co-conspirator in an indictment in order to fulfill any legitimate prosecutorial interest or duty. For purposes of indictment itself, it is sufficient, for example, to allege that the defendant conspired with "another person or persons known." In any indictment where an allegation that the defendant conspired with "another person or persons known" is insufficient, some other generic reference should be used, such as "Employee 1" or "Company 2". The use of non-generic descriptors, like a person's actual initials, is usually an unnecessarily-specific description and should not be used.

If identification of the person is required, it can be supplied, upon request, in a bill of particulars. See USAM 9-27.760. With respect to the trial, the person's identity and status as a co-conspirator can be established, for evidentiary purposes, through the introduction of proof sufficient to invoke the co-conspirator hearsay exception without subjecting the person to the burden of a formal accusation by a grand jury.

The prohibition against naming unindicted co-conspirators should not extend to persons who have otherwise been charged with the same conspiracy, by way of unsealed criminal complaint or information. In the absence of some significant justification, federal prosecutors generally should not identify unindicted co-conspirators in conspiracy indictments. See USAM 9-16.500; 9-27.760.

[updated April 2018] [cited in USAM 9-16.500]

https://www.justice.gov/usam/usam-9-11000-grand-jury#9-11.130