On Sunday afternoon, a man named Edgar Welch walked into the Comet Ping Pong pizzeria with an AR-15 rifle, firing at least one shot and surrendering only when he was satisfied that no children were being held captive on the premises. It was the result of a bizarre theory that has circulated on right-wing conspiracy sites for more than a year, alleging that the site was part of a pedophilia ring implicating top aides to Hillary Clinton. No evidence for the scheme has ever been produced, but it hasn’t stopped the theory from gaining support among some of the conservative world’s loudest voices.

The spread of that story has been fueled by some alarming new factors, including apparent promotion by foreign powers and the rise of misinformation sites on Facebook, but the basics are troublingly familiar. The theory recycles elements of everything from Disney’s LSD rumors to the ‘80s satanism craze. In each case, the conspiracy presents us with a seemingly normal scene, laden with symbolism and overinterpreted by committee. That formula is what made the Comet story so quick to spread, so hard to kill, and ultimately so dangerous.

A SEEMINGLY NORMAL SETTING

The essence of a conspiracy theory is a painful, maddening gap between the official story and the secret truth. Creating that gap requires a wild conspiracy, of course, but the everyday setting is just as important, whether it’s a preschool or, in this case, a pizza place. Comet Ping Pong is a completely inoffensive setting, which is part of why it’s been home to so many photo ops by politicians. It’s public, so anyone can walk right in. There are thousands of pictures that have been taken inside the establishment, many of which are available on the site’s public Instagram page. They show exactly what you’d expect — people eating pizza, hanging out. There would seem to be nothing to hide.

But if you see that inoffensiveness as a façade, mundane aberrations begin to look sinister, whether it’s an unusual Instagram or an off-color concert ad. The theories claimed the pizzeria’s basement was the site of the trafficking, so when the owner announced the pizzeria didn’t have a basement, that ordinary detail didn’t disprove the theory, it became unspeakably sinister, one more sign of a cover-up. Trying to kill the theory with contradictory evidence just fuels the fire.

AN OBSESSION WITH SYMBOLISM

One of the strangest parts of the Comet conspiracy theory was the fixation on a triangular spiral symbol, which adherents saw as proof of the pizza shop’s true nature. The premise was that the FBI had identified the symbol as a kind of pedophile hobo code, used to designate certain settings as friendly to child abuse. When the same symbol appeared in a nearby pizza shop’s logo, it was taken as powerful evidence of the shop’s involvement in the scheme. Armed with that evidence, groups eagerly pieced through Comet’s Instagram feed and interior decor in search of the symbol.

It’s a classic case of confirmation bias — look for something vague enough and you’re bound to find it — but the overall psychological effect is even more important. For believers in the theory, the symbol really does function as a kind of secret handshake, known only to insiders and revealing otherwise invisible schemes.

A FEVERISH HIVE MIND

Confirmation bias is particularly strong when there’s a lot of information and enough eyes to scan through all of it. When the group is already committed to a certain interpretation of events, it also lets them reinforce and expand that interpretation, pushing the most extreme ideas to the surface. In earlier iterations, that was confined to mailing lists or chat rooms, but now social networks let it sprawl across platforms, playing out on Facebook, Twitter, and conspiracy-friendly news sites.

That effect was particularly vivid when the group got a hold of emails leaked from Clinton campaign manager John Podesta. Simple references to pizza, cheese, and sauce (regarding, let’s remember, a pizzeria) were twisted into coded signals to a sex-trafficking ring. From the outside, it looked like nonsense, but once those conjectures hit Twitter or Facebook, tens of thousands of people shared them, reaffirming to each other that they had found proof of a sex-trafficking ring.

CHILD ABUSE

The core of all of it is child abuse, a crime that’s both plausible enough as a conspiracy and horrifying enough to supersede normal concerns about plausibility and decorum that might take hold if we were talking about tax fraud. Child abuse rings are very real, and in the wake of recent stories about abuse in the Catholic Church, at Penn State, and among Congressional pages, it’s not so implausible that a similar scandal could be brewing at a DC pizzeria, and many were clearly so horrified by the possibility that the evidence itself became an afterthought.

That disregard for evidence is a much more potent force than it was in previous decades. The effect is amplified by the internet — particularly Facebook, where it was easy for insiders to reach other insiders and share new “evidence.” It was also amplified by the election, which created a demand for ever-more-elaborate conspiracy theories concerning Hillary Clinton, only fueled by Clinton’s own penchant for secrecy. President-elect Trump and his supporters explicitly encouraged some of those theories, giving them even more legitimacy.

Still, the core of the story is something much simpler and more familiar, a kind of feverish distrust channeled into often arbitrary details, spread from person to person, feeding off the distrust of larger media institutions. What’s surprising is how powerful that cycle has become — and with Sunday’s attack, how dangerous.