According to no less a legal authority than Supreme Court Justice Antonin Scalia, the function of the grand jury is not to inquire upon what foundation the charge may be denied or to otherwise try the suspect’s defenses, but only to examine upon what foundation the charge is being made by the prosecutor. As a consequence, neither in this country or in England has the suspect under investigation by the grand jury ever been thought to have a right to testify or to have exculpatory evidence presented.
Furthermore, prosecutors work hand in glove with police in prosecuting their cases. They depend on their testimony, their evidence and their cooperation. There is an inherent bias favoring law enforcement by prosecutors. Prosecutors can manipulate the grand jury in any decision they want. Having prosecutors investigate the police is like having the fox guard the hen house.
I would suggest there has been a misuse, indeed an outright abuse of the grand jury system by prosecutors in the Michael Brown and the Eric Garner cases rising to the level of prosecutorial malfeasance. The only reasonable solution to this problem of the investigation police officers who kill unarmed citizens is to appoint a special prosecutor unencumbered by by bias, mutual obligation, and racial animus.