After the 2016 presidential election, there has been a lot of debate about the impact of fake news that widely proliferated through social media during the campaigns.

Now we know how at least one fake news story has manifested into something quite real: On Sunday, Edgar Maddison Welch, a 28-year-old from North Carolina, armed himself with an assault rifle and went into a pizzeria in Washington, DC, to, he told police, “self-investigate” one of the conspiracy theories perpetuated by fake news. He fired one or more shots in the restaurant, although no one was apparently hurt.

The conspiracy theory, known as “Pizzagate,” apparently connected Hillary Clinton’s presidential campaign to a nonexistent child sex ring at the Comet Ping Pong pizzeria, Faiz Siddiqui and Susan Svrluga reported for the Washington Post:

The restaurant’s owner and employees were threatened on social media in the days before the election after fake news stories circulated claiming that then-Democratic presidential nominee Hillary Clinton and her campaign chief were running a child sex ring from the restaurant’s backrooms. Even Michael Flynn, a retired general whom President-elect Trump has tapped to advise him on national security, shared stories about another anti-Clinton conspiracy theory involving pedophilia. None of them were true. But the fake stories and threats persisted, some even aimed at children of Comet Ping Pong employees and patrons. The restaurant’s owner was forced to contact the FBI, local police, Facebook and other social-media platforms in an effort to remove the articles.

Following the incident, Welch has been charged with assault with a dangerous weapon. Police also found two other firearms besides the assault rifle in the restaurant and one in Welch’s car.

The incident is terrifying. And as fake news continues permeating social media platforms like Reddit, Twitter, and Facebook, there’s a real concern that more similar events could take place.

Fake news is now widely recognized as a big problem

Fake news widely proliferated during the last few months of the presidential election.

Over at BuzzFeed, Craig Silverman pit Facebook engagement for the top 20 fake news stories — like “Pope Francis shocks world, endorses Donald Trump for president” (he did not) and “FBI agent suspected in Hillary email leaks found dead in apartment in murder-suicide” (this did not happen) — against the top 20 legitimate news stories, from outlets like the New York Times and Huffington Post. In the last three months of the election, the fake news stories got more Facebook engagement, meaning they got more shares, reactions, and comments.

Facebook at first declined to do anything about the proliferation of fake news, and even denied that it was much of a problem. But the social media platform now seems to be moving in another direction.

“For so long, we had resisted having standards about whether something’s newsworthy because we did not consider ourselves a service that was predominantly for the distribution of news. And that was wrong,” Facebook executive Elliot Schrag said at a panel in Massachusetts last week. “We have a responsibility here. I think we recognize that. This has been a learning for us.”

The bad news: Facebook doesn’t seem to have an idea about what it will do yet. (Check out Vox’s explainer on fake news by Tim Lee on why this will be a tricky problem to fix.)

Until Facebook does something, we’re left to wonder just how much of an impact all of this fake news will have not just on people’s ideas but, based on the pizzeria incident, people’s actions as well.

Watch: It’s now on America’s institutions — and Republicans — to check Donald Trump