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Several US schools to install pepper-spraying drones to respond to school shootings
(www.theguardian.com)
This is a most excellent place for technology news and articles.
There is really no other comparable country in the world with the same gun proliferation as the US..
So yes, guns are definitely the main factor here.
You can't assume a correlation from one data point. If it's the guns, then that'll show up in data from other countries too. When's the last time you heard of a mass shooting in Yemen?
Couple that with the fact that so many Americans wanting to shoot as many human beings as physically possible is still an enormous cause for concern even if we somehow prevent them from getting a gun.