The nation’s streets are filled with protesters shouting for justice after separate grand juries in new york and missouri declined to indict two white police officers who killed unarmed black men while on duty. the decision by the grand juries in those cases has raised questions about their validity, especially given their secrecy. grand juries can serve a variety of purposes at both the state and federal levels, said rebecca lonergan, associate professor at the usc gould school of law. in criminal cases, the grand jury can be a way for prosecutors to bypass the preliminary hearing phase of a trial. the prosecutor presents all the evidence to a grand jury, which — instead of a judge — decides whether there is enough evidence to file charges and send the accused straight to a trial.