Hello, I have a question about a situation that my family is in. In the county i'm in they have auxiliary police and struck and killed by them

The auxiliary police struck and killed my brother and the city is trying to cover it up and saying that he was in an accident and we have airtight proof that they struck and kill him for the civil suit and we gave them this proof and they went out and started to scare all of our witnesses out of testifying and threatening them that is what they told us (the witnesses). We told our prosecutor that we wanted them to get the maximum time in prison or ten years minimum, then our prosecutor said,"no we can only give him what I want, five years. With a mandatory 18 months, because I really don't have to ask you anything; I can do what I want." Also he has already accepted the 5 years with a mandatory 18 months. We really need help with this issue, thank you in advance.