It is a long-established and basic reality of law enforcement in America: Prosecutors who want an indictment get an indictment. In 2010 alone, federal prosecutors sought indictments in 162,000 cases. All but 11 times, they succeeded.

Yet the results are entirely different when police officers kill unarmed civilians. In those cases, the officers are almost never prosecuted either because district attorneys do not pursue charges in the first place or grand juries do not indict, as happened most recently in Ferguson, Mo., and Staten Island.

There are various explanations for this, but the most obvious is the inherent conflict of interest that exists for prosecutors, who rely heavily on the police every day. Cops arrest suspects; they investigate crimes; they gather evidence; and they testify in court, working essentially in partnership with prosecutors.

Whether or not bias can be proved in a given case, the public perception of it is real and must be addressed.