Pass a law to independently investigate acts of violence and shootings by police, and to prosecute them for illegal acts

Pass a law or an executive order that any Police action, including violence or killings against citizens, be investigated by an independent group, who is not associated with the offending Police Department in question, and that if the independent investigation shows that disciplinary actions or prosecutions should be taken, that those disciplinary and prosecutorial actions be taken with the appropriate amount of justice.