Police officers accused of crimes should not be investigated by their colleagues nor prosecuted by the district attorneys who depend on their investigation and testimony. In such situations bias is unavoidable, degrading public trust in the outcome of the process. This in turn degrades public trust in law enforcement in general, contributing to the cycles of hostility seen in Ferguson and elsewhere.
The only way to ensure that police officers are not given special treatment by the criminal justice system is for an outside department to handle all such investigations and prosecutions. Such a department would not be influenced by personal relationships or fear of reprisal, thus helping to restore objectivity to the process and trust in its results.
Quis custodiet ipsos custodes?