Columbine High School students cry while awaiting friends and family at Leawood Elementary school in Littleton, Colorado, April 20, 1999. (Gary Caskey/Reuters)

On another terrible day, I hate to introduce even more pessimism, but when we discuss mass shootings, one of the first questions we ask is the simplest and also the hardest to answer. Why? Why does this keep happening? Those who advocate for gun control have an immediate answer — the prevalence of guns in the United States. Yet guns have been part of the fabric of American life for the entire history of our republic. Mass shootings — especially the most deadly mass shootings — are a far more recent phenomenon.

Writing in 2015, Malcolm Gladwell wrote what I think is still the best explanation for modern American mass shootings, and it’s easily the least comforting. At the risk of oversimplifying a complex argument, essentially he argues that each mass shooting lowers the threshold for the next. He argues, we are in the midst of a slow-motion “riot” of mass shootings, with the Columbine shooting in many ways the key triggering event. Relying on the work of Stanford sociologist Mark Granovetter, Gladwell notes that it’s a mistake to look at each incident independently:

But Granovetter thought it was a mistake to focus on the decision-making processes of each rioter in isolation. In his view, a riot was not a collection of individuals, each of whom arrived independently at the decision to break windows. A riot was a social process, in which people did things in reaction to and in combination with those around them. Social processes are driven by our thresholds—which he defined as the number of people who need to be doing some activity before we agree to join them. In the elegant theoretical model Granovetter proposed, riots were started by people with a threshold of zero—instigators willing to throw a rock through a window at the slightest provocation. Then comes the person who will throw a rock if someone else goes first. He has a threshold of one. Next in is the person with the threshold of two. His qualms are overcome when he sees the instigator and the instigator’s accomplice. Next to him is someone with a threshold of three, who would never break windows and loot stores unless there were three people right in front of him who were already doing that—and so on up to the hundredth person, a righteous upstanding citizen who nonetheless could set his beliefs aside and grab a camera from the broken window of the electronics store if everyonearound him was grabbing cameras from the electronics store.

Gladwell then argues that Columbine changed the thresholds. The first seven of the “major” modern school-shooting incidents were “disconnected and idiosyncratic.”

Then came Columbine. The sociologist Ralph Larkin argues that Harris and Klebold laid down the “cultural script” for the next generation of shooters. They had a Web site. They made home movies starring themselves as hit men. They wrote lengthy manifestos. They recorded their “basement tapes.” Their motivations were spelled out with grandiose specificity: Harris said he wanted to “kick-start a revolution.” Larkin looked at the twelve major school shootings in the United States in the eight years after Columbine, and he found that in eight of those subsequent cases the shooters made explicit reference to Harris and Klebold. Of the eleven school shootings outside the United States between 1999 and 2007, Larkin says six were plainly versions of Columbine; of the eleven cases of thwarted shootings in the same period, Larkin says all were Columbine-inspired.

Here’s the most ominous part of the Gladwell thesis. The “low threshold” shooters are motivated by “powerful grievances,” but as the riot spreads, the justifications are often manufactured, and the shooters more and more “normal.” Here’s Gladwell’s chilling conclusion:

In the day of Eric Harris, we could try to console ourselves with the thought that there was nothing we could do, that no law or intervention or restrictions on guns could make a difference in the face of someone so evil. But the riot has now engulfed the boys who were once content to play with chemistry sets in the basement. The problem is not that there is an endless supply of deeply disturbed young men who are willing to contemplate horrific acts. It’s worse. It’s that young men no longer need to be deeply disturbed to contemplate horrific acts.

In other contexts, he’s elaborated further. The preparations for massacres are often extremely detailed. Shooters (and wannabe shooters) will often film videos, mimic the dress and poses of the Columbine killers, and otherwise copy the shooters who came before. Gladwell is hardly an NRA conservative — and he believes gun control “has its place” — but he also shares this grim warning: “Let’s not kid ourselves that if we passed the strictest gun control in the world that we would end this particular kind of behavior.”

Indeed, it’s the pattern of elaborate preparation and obsession with the subculture of mass shooters that has led in part to my own advocacy of the gun-violence restraining order. While we don’t have sufficient details about today’s shooter in Texas to know if it would have made a difference, it’s a fact that large numbers of mass shooters broadcast warning signals of their intent to do harm, and it’s also a fact that family members and other relevant people close to the shooter have few tools at their disposal to prevent violence. A gun-violence restraining order can allow a family member (or school principal) to quickly get in front of a local judge for a hearing (with full due-process protections) that can result in the temporary confiscation of weapons from a proven dangerous person.

While early reports are often wrong, there are indications that the Texas shooter engaged in behavior that sounds eerily like the Columbine shooting. We’ve seen reports of a trench coat, of the use of similar weapons, and of explosives — all hallmarks of the Colorado massacre. When I think of Columbine, I think of Gladwell’s essay. There are young men in the grip of a terrible contagion, and there is no cure coming.