Police are not able to police themselves. The Boston Globe found "The Boston Police Department almost never believes citizens who report misconduct or violence by police officers, and even when officers are found to have acted inappropriately, they often are not punished with anything harsher than an oral reprimand, according to a Globe analysis of more than four years of department discipline data." Sadly, an expert says this is "very typical of similarly sized departments."
Read the article: Within the Boston Police Department, complaints against officers are rarely confirmed or result in punishment