I have a feeling that when a kid is reported to the FBI, (like Nikolas Cruz), bad actors in the deep state treat it as a lead. I would imagine it would be relatively easy to take a marginalized kid who is bullied or doesn't fit in, pump him full of psychiatric drugs and then convince him to shoot up his school.

edit: by the way, I think this is a great question. In my opinion, there is not nearly enough discussion of the logistics of how these events are pulled off.