The point of investigations
When an incident occurs that requires investigation, the investigator is faces with many pieces of information which they need to filter to discover their relevance. This is an iterative process which draws the focus onto the correct target. This is true for any investigation, and any source of information, be it internet or otherwise. The difference between public safety investigations and any other is that the targets will not give their consent to be investigated, if they are known at all. The question you need to ask, is, how could it happen if public safety officers can't do this. They are always far too busy to follow up irrelevant leads.
All other investigations carried out for research by public or private organisations is consent driven, you give your consent to have your life examined to determine the colour of your next sock offer through your loyalty card or purchase contract, in effect because, no matter who you are the investigation's outcome is beneficial to both sides. Public safety investigations generally have a winner and a looser.
Biting the hand that feeds IT
