On Sunday afternoon, as families ate a late lunch at Comet Ping Pong in Washington, D.C., a man armed with a rifle walked in, aiming to “self-investigate” fake stories that the small pizzeria was a front for a child sex ring led by Hillary Clinton.

No one was hurt, but a shot was fired, families and staff fled and a swath of the quiet neighborhood was locked down for hours. The suspect, 28-year-old Edgar Welch of Salisbury, N.C., referenced what’s known online as “Pizzagate,” a fabricated story on dozens of fake news websites, which puts Comet Ping Pong at the center of bizarre, debunked charges of child trafficking. The hoax has been shared millions of times by Donald Trump supporters on social media including Reddit, Twitter, Instagram and Facebook.

That an insane online conspiracy theory brought violence to a neighborhood business five miles from the White House is mind-boggling. Even worse is that similar fake stories involving Mrs. Clinton and pedophilia have been promoted by Lt. Gen. Michael Flynn, President-elect Trump’s choice for national security adviser. A conspiracy-minded Islamophobe forced out as chief of the Defense Intelligence Agency in part for his lack of judgment, General Flynn lent credibility to the provably false charge by tweeting links to fake conspiracy stories cited by Pizzagate trolls. Mike Flynn Jr., Mr. Flynn’s son and adviser, and a member of the Trump transition team, was still spreading the Pizzagate lie after Sunday’s incident in the capital.

In the days before the election, General Flynn’s Twitter account bristled with all-caps exclamations about bizarre Clinton conspiracy theories. On Nov. 2, General Flynn tweeted: “U decide — NYPD Blows Whistle on New Hillary Emails: Money Laundering, Sex Crimes w Children, etc...MUST READ!” He included a link to a fake story on a website called “True Pundit.” Two days later, he tweeted a link to another false story, accusing John Podesta, Mrs. Clinton’s campaign chief, of participating in satanic rituals. This, from the man who would guide the United States’ first response to global threats?