The conversation between Mr. Korellis and Ms. Haas, which took place a decade ago, was the result of a specific kind of outreach: survivors reaching out to other survivors. More and more, those who have lived through mass shootings — which were defined by the Obama administration in 2013 as a shooting with three or more deaths — are connecting with new survivors to create support groups between communities.

In this case, Virginia Tech students heard about the shooting at Northern Illinois University, waited a few days and then reached out with cards and phone calls. Their message was simple: We’ve been through it, too. And we’re here to talk.

“We didn’t know who else to talk to,” said Mr. Korellis, 32, now a geographic information systems project manager for Walgreens. “But hearing about a different shooting from their perspective helped. It was like, ‘Tell me how the next year is going to be, what am I going to be dealing with?’”