I think the point is that this information wasn't brought to the attention of the authorities. And from my understanding he had no previous criminal record or anything of the sort. So while it might have been known among his friends that he could have done such a thing, it was either a threat that wasn't taken seriously, or people were scared into silence. On the day he showed the murder weapon to friends with a warning not to tell or else he would kill them. Obviously he was entirely capable of carrying out that threat and there's no doubt he would have done so. Can you blame those friends of his for saying nothing? They might have been scared stiff, and perhaps were still unable to believe that he was actually going to go through with it.
I don't think anyone can be blamed for missing this. Sometimes there is nothing that could be done to stop a tragedy.