a cops word is unquestioned in court (unless you have solid evidence), thats an amazing amount of trust, and many deserve it. but, with that type of trust should come equally serious responsibility.
IMO, in a case of a cop lying, testifying that he knows things he only believes, planting evidence in a criminal case or -if they cover for or keep quite while knowing that another does- there should be no statute of limitation, the sentence should simply be the double the maximum sentence that the accused faced. in this case double max sentence faced be each and every one of the accused.
they same formula should be applied with equal vigor regardless of the color of the cops or the accused's skin.