James Alefantis first learned about the conspiracy theory surrounding his pizzeria from a reporter’s phone call. In the waning days of the 2016 election, Will Sommer, then of the Washington City Paper in DC, had been following a thread on the site Reddit in which leaked emails from the Clinton campaign adviser John Podesta were used to justify an elaborate, and entirely false, story of a Clinton-run child abuse ring in the back of Comet Ping Pong pizza. Alefantis had never heard of the faux online hysteria and assumed, as he recalled in HBO’s new documentary After Truth: Disinformation and the Cost of Fake News, that “everybody is up about this election and passions are high – I’m sure this will go away in a couple of days”.

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It did not go away. The ludicrous theory, known as Pizzagate, was picked up and propagated by infamous conspiracy monger Alex Jones, and disseminated on Facebook and Twitter. Harassment of Comet and its staff escalated: one-star Yelp reviews appeared with references to child dungeons. Pictures of Alefantis posted to Instagram of his godchildren were repurposed to attack him as a pedophile, with specific, hateful references to his identity as a gay man. Prank callers issued death threats. And then, on 4 December 2016, Edgar Welch, a 28-year-old man from North Carolina, convinced from online forums that he had to find the children himself, entered the pizzeria with an AR-15.

No one was hurt and Welch was arrested, but the episode underscored the terrifying power of conspiracy theories and fake news articles, cultivated in forums such as Reddit and 4chan and propagated by Facebook and Twitter, to metastasize offline, with dire consequences. “I don’t think you can understand Pizzagate until you see that bodycam footage from the DC metro police that we have in the first 20 minutes of this film,” Brian Stelter, host of CNN’s Reliable Sources and an executive producer of After Truth, told the Guardian. “It shows how average Americans can be radicalized by insane content on the internet and can be motivated to get in a car with a gun and go confront people at a pizzeria.”

After Truth tracks the influence of disinformation – a deliberately disseminated falsehood, as opposed to “misinformation”, which is an unintentional factual error – from a niche topic in 2015 through Russian weaponization in the 2016 election and ubiquity in the Trump presidency. “We’re looking at some of the biggest lies that continue to manipulate people’s imaginations, even after they’ve been thoroughly debunked and clarified,” Andrew Rossi, the director, told the Guardian. Though the term “fake news” dates back to 2014 – when BuzzFeed News’s Craig Silverman popularized it to describe false stories about the Ebola crisis – After Truth begins in 2015, with online conspiracies around a military drill in Bastrop county, Texas. Known as Jade Helm, the theories, harnessed by YouTube personalities, spilled into the town’s real life – now eerie footage depicts an army spokesperson shouted down by people who dismiss his reassurances as propaganda; instead, they believed an internet personality-driven theory that underground tunnels connected the town’s Walmarts to the military base. The hysteria caught Texas governor Greg Abbott’s attention, who treated it seriously. Russia took notice, and started replicating the pattern heading into America’s election year.

Beginning the arc in 2015 was deliberate – “this is bigger than any one president or term in office,” said Stelter – but disinformation certainly ramped up in unison with Donald Trump’s rise to the Republican nomination and, eventually, the presidency. His weaponization of the term “fake news” against legitimate news outlets in January 2017 represented “a signal moment”, said Stelter. “The president took the term and exploited it for his own political gain. Now, it’s the world’s worst game of whack-a-mole.”

After Truth seeks to put a face on the generally amorphous, sterile concept of disinformation, whether that be the pizzeria owner hounded by internet trolls or the frustratingly human role of prejudice and hate as “a foundation for fake stories and as a magnet for people who feel aggrieved to channel their confusion with the world and their disappointment into demonizing a group”, said Rossi. Comet Ping Pong was targeted as an LGBTQ+ safe space, for example; the Barack Obama birtherism theory in the early 2010s, of which Trump was a vocal proponent, was transparently racist.

Facebook Twitter Pinterest James Alefantis, owner of Comet Ping Pong. Photograph: Courtesy of HBO

For Comet, there was a somewhat happy ending: loyal customers rallied around the business, keeping it open. The same cannot be said for the family of Seth Rich, a 27-year-old Democratic National Committee (DNC) staffer killed in an unsolved botched robbery in the summer of 2016. Rich’s death became fodder for online conspiracy theorists, who baselessly linked the staffer to DNC emails actually leaked by Russia and claimed murder by the Clintons. The Fox News host Sean Hannity ran a primetime story boosting the Rich conspiracy, which the network later retracted (Hannity never apologized). Aaron Rich, speaking with Rossi, says fighting incessant trolls has left him unable to grieve his brother, even three years later.

Whether it’s Pizzagate, activating people into believing an outlandish pedophilia ring must be stopped, to Rich conspiracy theorists who feel they have access to privileged information, After Truth explores how much of disinformation’s power lies in human emotion. “Fake stories are often successful when they capture their audience’s hearts,” said Rossi. “What we’re trying to do in the film is to provide a different story that also appeals to people’s hearts, which is the pain and the devastation that’s caused to the victims of these stories.”

Putting a face on disinformation, however, means also looking at its perpetrators; After Truth includes interviews with or footage of noted conspiracy theorists, hucksters or far-right media personalities, including Jones and Jack Burkman, a notoriously incendiary Washington lobbyist who hosted a press conference in 2018 with bad-faith provocateur Jacob Wohl, meant to smear Robert Mueller with a sexual assault allegation that fell apart, somewhat thrillingly, on Rossi’s camera as the “victim” elects not to show up.

Talking to figures who believe all press is good press has been “the third rail of the project, and has been in the conversation among documentary film-makers who want to tackle this topic,” said Rossi. “How do we do this without giving oxygen to the liars and propagandists?” For Rossi, it was partly having someone like Burkman admit his cynicism, as in a 2017 interview in which he called disinformation a “tool of war”, like chemical weapons – if someone else can use it, he says, I may as well use it too. “And then also to combine observational film-making with context, to immediately rebut a lie and debunk it in the moment.” Journalists such as Silverman, CNN’s Oliver Darcy, Recode’s Kara Swisher and the New York Times’s Adam Goldman debunk false claims in real time and add crucial context to the film’s scope and timeline. “It’s a delicate balance to strike,” said Rossi, “but always make sure it’s not misleading and the truth is very clear.”

Ultimately, according to Stelter, when it comes to dispersing or de-escalating disinformation, “there’s no one solution, there’s no one right answer”. It’s understanding media literacy 101, it’s legislating responsibility for massive platforms of information, Facebook first among them. And in the midst of a once-in-a-generation pandemic in which correct, reliable information and public trust are now literally a matter of life and death, an understanding of the tools of disinformation may be more important than ever. “Disinformation in the sense that our information world is polluted is something we all have in common,” said Stelter. “It’s something we all share, and it’s a problem that we all collectively have to address.”