Four years ago today, 20 children and six staff members were killed when a gunman stormed Sandy Hook Elementary School in Newtown, Connecticut. It was one of the deadliest school shootings in U.S. history. And it’s the day Nicole Hockley’s life was forever changed: Her 6-year-old son, Dylan, a first-grader, died in the arms of his special-education assistant. Hockley’s elder son, Jake, now 12, survived, but she has said it was the day “he discovered some monsters are real.”

Hockley has since become an example that a mother’s love is a powerful force: Three days after the massacre, together with others who lost loved ones in the shooting, she cofounded Sandy Hook Promise, an organization to prevent gun violence, both through fighting for stricter gun laws and by focusing on mental-health education. It has gone on to educate an estimated 1.5 million students, administrators, and parents through its training programs. Sandy Hook Promise’s new “Know the Signs” campaign, punctuated by a powerful new PSA, “Evan,” teaches adults and children how to recognize the warning signs of a school shooter before violence occurs (more on those signs below).

In the four years since the Sandy Hook tragedy, Hockley has gone from a stay-at-home suburban mother to one of the leading voices in the politically fraught movement for gun reform. She was standing behind President Obama earlier this year when he announced his executive action on guns. She spoke with Vogue.com about her ongoing mission, whether the upcoming Trump administration will impact it, and how she will mark the fourth anniversary of her son’s death.

Sandy Hook Promise is raising awareness about the warning signs of a shooter, or someone contemplating suicide. What are some of those signs?

Some of those signs are excessive bullying and chronic social isolation, that perception, whether it’s real or perceived, of being persecuted by others. Any sort of studying shootings or other harmful activities, or giving away possessions, as in the case of suicide. Certainly any overt threat that is spoken, written, posted, tweeted. That is actually something that we have seen from previous shooters—they put it out there, but no one is taking it seriously. One sign on its own doesn’t necessarily mean that someone is at risk of violence, but if we see several warning signs together, this is about getting help.