I think that most people who need to know, know why the SIB, Police, MOD Plod etc etc were involved in this tragic accident. If you don't know why they were involved then you don;t need to know.
Unless there was some (unlikely) form of enemy or terrorist activity involved, this sounds suspiciously like some sort of cover-up to me.
I have witnessed first-hand strenuous attempts to cover up cockups, incompetence, dishonesty with the "need to know" argument, and in one case with the issue the far more insidious and fatuous "Public Interest Immunity Certificate".