External investigations

I don't know if that has been brought up or not, but I want to mention the fact that law enforcement agencies as they are, "investigate" themselves when issues are brought up, and no outside source is allowed to conduct an investigation for them.
This seems to be contradictory to be in favor of we the people, as there should be an outside investigation team to determine if what was done was proper, and according to the supreme law of the land. There have been way too many instances where common sense alone would tell us that certain acts were a severe case of improper justice based on the "internal" investigations that have taken place for many incidents.
Just curious if this is in the discussions or not...