for a lone person to pull off.

Stockpiling ammo and guns is a piece of cake. Transporting them in a town where there is a gun show wouldn't really raise any eyes. The vast majority of mass shooters either commit suicide themselves or commit suicide by cop the moment they are confronted.

The act of mass shooting is about power. Much like rape is about power and not sex, mass shootings are about power and not murder. As soon as getting caught is imminent, the power is gone and so is the reason for living. Committing suicide isn't unusual, it is to be expected.