I believe you are right. To me, and admittedly without many facts this last week or so, the ONLY thing that makes sense to me now is terrorism of some sort.
Instead of checking for the gun ... how about tracking the sales of a large caliber, long gun silencer. Nobody hears the crack of report to gauge direction ... the guy next to you just falls.
The Silencer peculiarity is a winnowing of sorts. So, it's the Silencer bought by a guy wearing Brown Dress Slacks who's nice enough, but doesn't get out much and keeps to himself - a quiet guy.
Pick him up boys, my work is done.