The man arrested Sunday for walking into the Comet Ping Pong pizzeria in Washington, D.C., and firing an assault rifle told police he was there to “self-investigate” Pizzagate, a grotesque alt-right conspiracy theory involving Hillary Clinton aides operating a child sex ring out of the restaurant. No one was hurt in the incident, but it underscored the very real danger of the politically charged lies that have proliferated online amid the rise of Donald Trump.

Trump himself is a well-documented conspiracy theorist, and the son of his incoming national security adviser, Michael Flynn, spread Pizzagate on Twitter after Sunday’s incident:

Until #Pizzagate proven to be false, it'll remain a story. The left seems to forget #PodestaEmails and the many "coincidences" tied to it. https://t.co/8HA9y30Yfp — Michael G Flynn🇺🇸 (@mflynnJR) December 5, 2016

(Contrary to a Washington Post report on Sunday, Flynn Sr. has not promoted Pizzagate—but he has proven susceptible to other Clinton conspiracy theories.)

Sunday’s incident is proof that even the most absurd stories online can inspire violence in the real world. “What happened today demonstrates that promoting false and reckless conspiracy theories comes with consequences,” Comet owner James Alefantis said in a statement. “I hope that those involved in fanning these flames will take a moment to contemplate what happened here today, and stop promoting these falsehoods right away.”

It’s fortunate that no one was physically hurt at Comet Ping Pong, but plenty of damage has been done. A beloved establishment may no longer feel quite as family friendly to many customers, and indeed an entire block of businesses—a bustling stretch that serves as the neighborhood’s main street, both commercially and socially—has been terrorized by vicious lies.