In the US law, indictment of charges is done by the jury but prosecutors under the law can request change in the jury, leading to formation of a grand jury or change in gender representation in the jury and thus they can also seek indictment of new charges against an accused or more than one accused in a case.
In the US law, the process of investigation continues even after indictment of charges formally by a jury against an accused, and whenever fresh evidence is found, the prosecutors or government lawyers can request for indictment of new charges, called Federal Superseding Indictment.
The need for Federal Superseding Indictment can arise on finding new evidence or more evidence in a case by the government investigators, and the laws allow them to seek new indictment of charges against the accused to ensure the trial is held in a fair and impartial environment.
The indictment of new charges is sought in order to bring more relevant sections of laws into act during proceedings of the case. For example, if a person is charged with a murder of a woman, he might be charged of rape and then murder in the superseding indictment, and it needs to bring into act the laws that are related to murder and rape. This also needs to change in the jury formation structure and also the number of members in a jury might also be raised.
There is no limit to how many times the government can seek for Federal Superseding Indictment, and since the investigation into collecting evidence of a case continues even after indictment of charges against an accused, the finding of fresh evidence might lead to request by the government lawyers for Federal Superseding Indictment more than one time. The law allows this and does not bar the prosecutors from finding something new and fresh that can affect the course of the trial.