In 1999, social media as we now know it did not exist and 24-7 news was a niche commodity. When Harris was plotting the Columbine massacre, he thought of his project in what now seem like antiquated Hollywood terms, recording in videos that he wanted “a lot of foreshadow­ing and dramatic irony” and debating with Klebold whether Steven Spielberg or Quentin Tarantino would better handle their story. Contrast that with Vester Lee Flanagan II, the troubled TV reporter who fatally shot two former colleagues, a reporter and a cameraman, in Virginia during a live broadcast last August. After the shooting, Flanagan detailed his motives on Twitter and uploaded a video of the murders, which he had filmed with his phone as he committed them, before killing himself.

On YouTube, you can find a split-screen video syncing Flanagan’s phone footage with the cameraman’s live feed, allowing the viewer to witness the moment simultaneously through the eyes of both the murderer and the murdered. That diptych is the apotheosis of the postmodern real-time violence introduced at Columbine, macabre evidence that terror is as easily packaged and consumed as any other form of content. In a country containing hundreds of millions of guns and smartphones, all of us are potential victims or digital bystanders in waiting — or both, in the case of Amanda Alvear, who Snapchatted some of the first shots fired at Pulse before she, too, was killed.

The active shooter has always seemed a particularly — if not exclusively — American menace, a dark confluence of the country’s mania for guns, self-­definition, reinvention and fame. This is part of what makes the shooting rampage’s belated adoption by self-­styled Islamic State martyrs — even American-­born wannabes with limited or nonexistent links to the aspirant caliphate — more jarring, in its own way, than any suicide bombing or airplane hijacking. As Adam Lankford, a University of Alabama criminal-justice professor who studies mass shooters and suicide bombers, points out, this development may be simple pragmatism: This is a country, after all, in which an AR-15 can be bought in some states in a matter of minutes by virtually anyone with valid identification, but a bulk order of ammonium-­nitrate-­based fertilizer may well produce a visit from the F.B.I.

But it’s also true that the propaganda that the Islamic State addresses to the West — the beheading videos, the martyrs’ testimonies, the social-­media exhortations to potential lone wolves — has always owed less to the world-­historical ambition of Sept. 11 than it does to the awful intimacy of Columbine, an act of killing that fell far short of its architects’ ambitions but haunts us all the more for having bound us to it as distant witnesses. The active shooter is never far away here; there are too many guns in America for us to ever delude ourselves into thinking otherwise. And the footage is always streaming, optimized for our many platforms.

The writer William S. Burroughs — who, having drunkenly killed his wife with a pistol, knew of which he spoke — once observed that “no one owns life, but anyone who can pick up a frying pan owns death.” The active shooter owns something that, in a distraction-­addled world, is perhaps even more coveted: our attention. This is certainly what Mateen wanted. In the final moments of his life, he reportedly stopped shooting long enough to search Facebook for responses to his own performance.