Imagine every major city, hospitals, schools, etc… all over the country have been doing these type of drills annually so that the authorities are actually prepared in the unlikely case it does happen in their jurisdiction. Then you have a tragedy like this happen, you trace back and, BAM, look a drill happened here (or near hear) within the year.
Well, DUH….of course. You just used confirmation bias to prove that choosing a random location in a major area (one that routinely has masses of people gather regularly and are “no-gun” zones) is a great target for a psycho to shoot up at will, with little to no resistance. But, isn’t that why they are selected to have these drills in the first place? Oh yeah.
It’s not a surprise that you might find a quarter on the ground under a street light at night, when they are strewn over the road in the dark. You find the ones illuminated in the light, but those in the dark go unnoticed.