Yet it’s striking that these alienated and hostile young men had parents who knew something was terribly wrong. So why couldn’t they stop the carnage?

Experts say that is a very complex question. In the first place, their sons were adults with agency — they could do what they wanted, and couldn’t be taken into custody unless proven to be a danger to either themselves or society. That’s an intricate, intensive process, even with alarming online evidence.

In the second place, according to Jack Levin, a professor of criminology at Northeastern University, and co-author of the recently published ‪Extreme Killing: ‪Understanding Serial and Mass Murder, “if you’re intervening to stop a murder, you’re already too late.”

The roots and reasons for mass killers go back to childhood, where a kid is “troubled but not yet troublesome” Levin told me in an interview. Parents should face the trouble early, and get help for their kids. Easier said than done.

Levin then said something that surprised me: “I’ve studied mass killers for more than 30 years and the vast majority of them do not suffer from a serious mental illness.”