Earlier quoted context omitted.
In a country of 330,000,000, people who have shot up schools may be around 330 or 0.0001%. It is impossible to assign any single causality to events that have such low occurrences unless you have billions of examples. (Yes, the rest of the world data can provide counterfactual data, but it is still mathematically impossible because there are 100s of variables at play)
It is possible and airliner crashes show that. They happen even less often and almost every single one had a precisely defined cause that was discovered after rigourus investigation. The reason why we do this despite the events having small chance of occurance, is because the impact is enormous and we give it as much scrutiny as we possibly can to prevent it from happening again. The same level of scrutiny is not hap…
All this to say, this is a hard problem, composed of thousands of overlapping adjacent elements, we have very little insight (or reliable insight-gathering mechanisms) applicable to the system under investigation, and so throwing money or effort at the investigation is a gamble that a feasible solution exists at all, rather than a process of concrete deduction with a deterministic endpoint. Maybe the optimal amount of scrutiny should be different.