A story at Fox News quotes a classmate of Thomas Crooks - the Trump shooter - as saying there was a prior threat by Crooks’ friends “to shoot up our school.” Following that threat Crooks “didn’t return to school for a few days.”
Sounds like another “known wolf” troubled youngster. Unfortunately there is probably a kid or two like this in most schools, adolescence is no walk in the park.
Most such don’t go beyond fantasizing. Now and then one acts out as Crooks did. We aren’t very good at predicting which will act out.