The cat is completely out of the bag on where to start with police brutality in America. The police unions in America are too big and give law enforcement too much power of civil rights. Start with the Police Officer Bill of Rights that is held to high standards in most American states.
The mere idea that police have a minimum of 24 hours to report on the specificity of all respective occurrences. This is topped with the practice of how Confidential Police Investigation records are expunge and kept away from public review.
Criminal court holds police above the law with qualified immunity where unless you bring charges against an officer that have been heard before the court before (a precedent) then it is not heard and thrown out.