Before embarking on a racist rampage in Buffalo on Saturday, the alleged gunman left behind a document denying membership in “any organization or group.”

“I am the sole perpetrator of this attack,” he wrote.

But the 180-page screed, which authorities are scrutinizing in connection with the massacre, leaves little doubt that the alleged perpetrator, 18-year-old Payton Gendron, belongs to a global fraternity fused by the Internet and fixated on the idea that White people are being intentionally replaced.

That idea, once relegated to the fringe, has gained currency on popular right-wing television programs and in the halls of Congress. The theory, known as the “great replacement,” has turned white nationalism into an international call to arms. The apocalyptic vision has accumulated followers during the coronavirus pandemic, which has deepened political polarization and accelerated the online flow of racist ideology.