This is speculative but the question must be asked in public and then must be answered by the authorities entrusted in regulatory duties. No it is not about guns!
Michael Moore - Reveals the real cause of Columbine. (watch minute 1:30)
... Eli Lilly for nearly 15 years covered up their own internal investigation that anyone on Prozac is 12 times more likely to commit suicide than on other anti-depressant.
See also this:
Oregon: Another Mass Shooting, Another Psychiatric Drug? 35 School Shootings/Mass Stabbings Tied to Psychiatric Drugs
According to an article posted in the Oregonian, “There are a number of indications that Mercer had mental health or behavioral issues. His screen name on some social media sites was ‘lithium love.’ Lithium is a psychiatric medication.
The question is what a particular drug or a class of drugs may be triggering it?