If there is an active FBI investigation, and the subject attempts to buy a gun and is denied because he or she is flagged, does that not alert the subject to the fact they are being investigated? What if the person is innocent? Would they have to prove their innocence in order to regain their second amendment rights? That's not how our system works. In out system, you are innocent until proven guilty.
We have to understand what our enemy does not-- that every human life is sacred, not just those we approve of, or those we can relate to, but all lives.