It didn’t take long after the arrest of a gun-wielding man at a District pizza restaurant on Sunday for the usual conspiracy theory that swirls around such an incident to percolate on social media and in the nether corners of the Internet.

The gunman, claimed the baying hounds of paranoia, was part of a “false-flag” operation — that is, he was an actor in an elaborate plot designed to discredit those who have for weeks spread a bizarre story about the restaurant being the locus of a child-molestation ring run by Hillary Clinton.

There’s no evidence that the suspect, identified as Edgar Maddison Welch, was acting on anyone’s behalf other than his own when he allegedly fired an assault-style rifle in the restaurant, Comet Ping Pong. There’s also nothing to indicate that any government or political party had conspired with him so that he’d take the fall for his alleged actions.

But saying so won’t stop the burgeoning industry of “false flag” wavers from saying otherwise.

False-flag conspiracies — the name comes from a military deception in which an enemy is tricked into thinking an opposing force is friendly — have a long and complex history. Historians have long debated, for example, whether the fire that destroyed the German parliament building, the Reichstag, in 1933 was set by the Dutch communist executed for the crime or was really a false-flag operation by Adolf Hitler’s Nazi party to further entrench its power.

Edgar Maddison Welch, 28, walked into D.C. pizza restaurant Comet Ping Pong armed with an assault rifle on Dec. 4, apparently to "investigate" a fake internet conspiracy. Here's what we know about him so far. (Deirdra O'Regan/The Washington Post)

False-flag claims have become barnacles on many of the most traumatic events of the past 60 years: John F. Kennedy’s assassination; the Oklahoma City bombing; the Boston Marathon bombing; the mass shootings at Virginia Tech, San Bernadino and Sandy Hook; and especially the terrorist attacks of Sept. 11, 2001.

In each case, the conspiratorial notion is that the “official” story conveyed by the government and the news media is a deception, all to justify some perfidious official action. The Sandy Hook shootings were supposedly staged to justify more gun control; 9/11 “truthers” believe the government, not terrorists acting on orders from Osama bin Laden, destroyed the World Trade Center as a pretext for war or to expand the government’s powers.

It’s not known how many people believe such claims, but they are widespread on the Internet. A quick search of YouTube on Monday turned up 785,000 videos labeled “false flag.”

False-flag allegations are a subset of the fake-news phenomena, but with a key difference. Whereas fake news comes out of nowhere, usually concocted wholesale, false flags are typically an attempt to explain or pin the blame after an actual event has occurred.

This is what makes the false-flag claims surrounding “Pizzagate” — as the supposed evils surrounding Comet Ping Pong are known — even more bizarre and confusing than usual. The allegations of child abuse, spread by dubious websites, are themselves unsubstantiated; now, on top of those accusations come an additional conspiracy claim that a false-flag operation was at work to discredit and censor the alternative sites that originally pushed the story.

Among those perpetuating the Pizzagate meme are Alex Jones, the proprietor of Infowars, a one-stop shop for conspiracies and false-flag claims. As a candidate, Donald Trump appeared on Jones’s syndicated radio program and praised Jones for his “amazing” reputation.

The story has also been pushed by Michael Flynn Jr., the son and sometime adviser of retired Lt. Gen. Michael Flynn, Donald Trump’s pick for national security adviser. “Until #Pizzagate proven to be false, it’ll remain a story,” he tweeted on Sunday.

Comet Ping Pong customers came out to support the restaurant after a gunman entered it with an assault rifle, firing it at least once. Several other businesses on the block have received other threats as well. (Whitney Shefte/The Washington Post)

Flynn linked to the Twitter account of Jack Posobiec, who describes himself as the special projects director of a group called Citizens4Trump. Among his many comments on the subject, Posobiec tweeted, “False flag. Planted Comet Pizza Gunman will be used to push for censorship of independent news sources that are not corporate owned.”

Like all conspiracy theories, evidence and facts to the contrary are usually worse than meaningless to the people pushing them, says Joseph Uscinski, author of “American Conspiracy Theories.”

“Conspiracy theories are unique theories because the evidence against them works in their favor,” he said in an interview. “If you believe there’s a group plotting in secret against you, how can you be proven wrong?

“People are going to believe what they want,” he said. “You can’t win with people who think the Democrats are running a baby cannibalism ring out of a pizza restaurant. You’re not going to convince them with the facts. They’re not reading The Washington Post.”

While Pizzagate may defame Hillary Clinton, Uscinski, a political-science professor at the University of Miami, says conspiracy theories are widely held and propagated. Liberal 9/11 “truthers,” for example, will view the destruction of the buildings as a Republican-led plot to provoke a war, whereas libertarian-leaning conservatives will view it as a secret government ploy to enact laws curtailing civil liberties and expanding government power. “It’s not like one party has a monopoly on viewing the world conspiratorially,” he said.