A friend of mine was killed, and I don't know the result after investigation. What department should I ask?

Better ask their home first. Maybe some things in the survey results are only made public to relatives first, so it is not convenient to make public. It's no use asking them yourself. You should also consider the feelings of your friends and family and see if they are willing to let you know now. If yes, you can ask where it was investigated. Criminal cases go to the Armed Police Brigade.