WASHINGTON — On any given day, locals flock to Comet Ping Pong, a pizza joint here not far from where I live, to eat, talk and, of course, play Ping-Pong. But last Sunday, a man armed with a military-style assault rifle and a pistol turned up for an entirely different reason: to see for himself whether the restaurant was indeed, as right-wing fake news reports and conspiracy websites have declared, the hub of a vile child sex-slavery ring masterminded by Hillary Clinton.

The absurdity of this story would be laughable if it hadn’t led a man to bring a rifle to a restaurant filled with families. And if it hadn’t resulted in an army of online terrorists harassing the owner, his employees and others along that block of Connecticut Avenue, accusing them of unspeakable crimes and even issuing death threats.

I’ve seen my share of conspiracy theories. My grandfather, who accidentally took a home movie of President John F. Kennedy’s assassination — now known as the Zapruder film — was implicated in some of the most delusional stories about that event: He had colluded with the C.I.A. to allow his film to be altered just days after the assassination; he had secret ties to Lee Harvey Oswald through a co-worker who later married Oswald’s close friend; and, wait for it, he was the one who pulled the trigger through an elaborate gun-as-camera mechanism at the bidding of the Jewish Mafia.

The government’s failure, in the historian Art Simon’s words, to come up with “a coherent and believable account of the assassination” left many gaps to be filled. While early assassination researchers performed a valuable function by making important information public, later conspiracy theorists relied on association and innuendo and cherry-picked details to build increasingly wild narratives.