“From my perspective, YouTube is perhaps the most troubling platform we have out there right now,” Danah Boyd, founder of the Data & Society Research Institute, told us.

What we found in Brazil went far beyond anything we had anticipated, with important — and sometimes disturbing — lessons for us all. Our findings appear as an article in today’s paper and as a half-hour episode this week of The Times’s new TV show, “The Weekly.”

As we were finishing our story, we heard news of the attack in El Paso, with its echoes of the March attack in Christchurch, New Zealand, also by a white nationalist apparently radicalized online. It reminded us that, just as radicalization can affect societies broadly, so can the harm it causes.

Statistically, mass shootings make up only a small fraction of America’s thousands of gun deaths; attacks with clear ideological motivations, such as the white supremacy and violent misogyny that have become hallmarks of online radicalization, are a mere fraction of those.

But they cause harm that goes far beyond the impact of bullets on flesh. One study found widespread trauma symptoms in the Norwegian public after the 2011 Norway attacks, particularly among those who had been physically near the attacks or felt a psychological identification with the 77 victims. Other research found a similar effect among New Yorkers after the Sept. 11 attacks.

Mass shootings that target the quotidian places where people go about their lives — schools, universities, places of worship, concerts, shopping centers and exercise studios — have systematically stripped away the sense that anywhere is safe. Anyone can feel psychologically close to a shooting or its victims. Anyone can be within the zone of trauma.