In cases where they do, the murderer is more likely to be someone you know then a stranger.
Of course the idea that you can not defend yourself against someone you know is silly. Just because I may have met someone does not move them into the benign category.
Yes, it means the authorities do no know who the murderer is, and therefore, they do not know the relationship of the murderer to the victim.
Murders where there is no connection between the murderer and the victim are usually the hardest to solve.
Nameless Gang Member #1 knows Nameless Gang Member #2 and that’s an “acquaintance”. These numbers are way off as UCR reports as much as 89% per year are gang/drug murders.