For those who can't believe police might not want to investigate these sorts of things, there was recently a knockout attack on a man and his girlfriend in Charlottesville and the girlfriend even got cell phone pictures (fairly clear in fact) of the attackers, yet after submitting a report and those pictures to the local police department there, they called back a week later to provide some additional information only to learn the case had never been assigned to an investigator. The only reason it has been now, I think it's safe to say, is that the local media became aware of that fact and started asking why. The police spokesman I saw on the video was providing what, to my ear, sounded like some pretty lame excuses of why no investigator had been assigned.
And again, this sort of thing is incredibly common for those sorts of attacks. Even large scale attacks involving 20 or 50 or even a couple of hundred people (which are common) - read Colin Flaherty's book on this, he lists not only hundreds of such incidents, but for many of them goes into the police's lack of desire to pursue them. I seem to recall he even has some police officers talk about why that is the case, under condition of anonymity.