The government's indictment of Zacarias Moussaoui names 23 "unindicted co-conspirators," including Osama Bin Laden and the 19 dead hijackers. What is an unindicted co-conspirator?
A co-conspirator is someone prosecutors believe entered into an agreement with at least one other person to break the law. Being unindicted just means the person hasn't been charged with a criminal offense. Co-conspirators can be unindicted for several reasons: They may be cooperating witnesses for the prosecution, or the government may not have enough evidence to convict them. Or they may be charged in another case. Or, as in the case of the hijackers, they can be dead, forgoing the need for an indictment. (Or, as in the case of Richard Nixon, they may be the president, and there may be doubt that a sitting president can be indicted, so you defer the matter to Congress.)
Most likely, the prosecution has not indicted Bin Laden and his top lieutenants because the government intends to try them before military tribunals. A formal indictment in a U.S. court might give Bin Laden and al-Qaida members grounds to claim they have the right to be tried in civilian courts.
So why name them on the indictment? Probably because the prosecution will want to use out-of-court statements made by the co-conspirators. Normally, out-of-court statements are considered hearsay. But under the federal rules of evidence, a statement is not hearsay if it is made "by a coconspirator of a party during the course and in furtherance of the conspiracy" (one of several hearsay exceptions). In order for a judge to admit that kind of statement into evidence, the prosecution must prove by a preponderance of the evidence that the person making the statement was a member of the conspiracy. (That burden of proof—"a preponderance of the evidence"—is lower than the "beyond a reasonable doubt" standard that a jury needs to convict someone.) There's no requirement that the unindicted co-conspirators be named in the formal indictment to get co-conspirator statements into evidence, but doing so may increase the likelihood that a judge will admit the testimony.
Explainer thanks David Robinson Jr. and Stephen Saltzburg of GeorgeWashingtonUniversityLawSchool.