ID of the people behind this is typically a multi-step approach. If the harrassment is life threatening, then your first stop should be law enforcement.
If less than life threatening, then usually the approach is to try and at least digitally ID the person via their IP address. There is some ways to get their IP address via investigations and that will then tell you the general location (city/state) of where they are from. For some people, that is enough to ID the person.
If not, often people in your situation have suspects in mind but just cant prove it. There are also ways to compare the IP address obtained to that of known potential suspects. This total process is not inexpensive but if your problem is serious enough, it may be the best (and only) solution. The guys over at cyberinvestigationservices.com can help you with this.
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