TRAUNSTEIN, Germany — The rumor began, as so many do these days, on Facebook.
A group of Muslim refugees in southern Germany, it was claimed, had dragged an 11-year-old girl to a pedestrian underpass and raped her. When the police denied the claim, it was said that politicians beholden to the European Union had ordered them to cover up the assault.
The rumor proved unfounded, but it provoked waves of fear and anger as it was pushed out across Germany by Facebook’s News Feed. Users worked one another up into fits, concluding that these dangerous refugees, and the duplicitous politicians who shielded them, would all have to be thrown out.
In most of the world, rumor-fueled meltdowns are taken as a fact of life, a product of Facebook’s propensity for stirring up peoples’ worst impulses.
But Andreas Guske, a trim, steely-eyed police inspector in the refugee-heavy Bavarian town of Traunstein where the rumor circulated, didn’t think his community could afford complacence. Attacks on refugees were already rising. And southern Germany is a front line in Europe’s battle over identity and immigration.
“Facebook is not just like a pinboard where people hang things and others read them,” Mr. Guske said in an interview. “Facebook, with its algorithm, influences people.”
And rumors, noted Mr. Guske, head of the police department’s communications office, travel faster in the age of social media, often growing more dangerous as they spread.
“That was not the case before Facebook,” he said.
So the police inspector and his two deputies set to work. Their goal: Through shoe-leather police work and door-to-door outreach, they would eradicate the rumor online and off, taking it as seriously as a pandemic or a new street drug.
Mr. Guske’s team members do not arrest people who post inflammatory rumors, or fine them. Instead, they act more like public health workers, inoculating communities from viral misinformation and its consequences.
Their efforts reflect rising concern among governments that Facebook is spinning up violence and extremism. They also reflect a lack of faith in the company to address the problem.
“We see that peoples’ feelings of safety and their actual safety are moving apart, like a pair scissors that are opening,” Mr. Guske said. “We’re trying to close those as much as we can. Hate can arise from this, which we’ve seen can turn the population against migrants and refugees.”
A Facebook spokeswoman said the company had been “working closely with German authorities” and had “trained hundreds of German officers over the past years on how to use our tools.”
The company says it has taken steps to curb hate speech, instituting special rules against anti-refugee posts. It also works with fact-checking organizations, including in Germany, to debunk false information on the platform.
Facebook enlists thousands of moderators to scrub posts that break its rules. It is a scalable solution that fits the company’s business strategy. But it does little to address Facebook’s real-world impact or to curb hate and misinformation that fall within the rules, like many of the rumors making the rounds in Traunstein.
Mr. Guske takes a more on-the-ground approach.
To stop the rumor about the supposed rape of the young girl at the underpass, Mr. Guske started by identifying local residents who had shared it on Facebook. Then he traced the story as it jumped between the social network and offline rumor mills — a reminder that Facebook moderators, who monitor only the platform itself, cannot stop misinformation’s spread into the real world.
Mr. Guske had two goals: to persuade the rumor’s progenitors to publicly disavow their claims, and to find whatever kernel of truth had grown into the tall tale. Showing local residents how Facebook had distorted reality, he believed, was the only way to persuade them to reject what they had seen there as false.
The rumor began, they found, when the police arrested an Afghan citizen accused of groping a 17-year-old girl two towns over, and pulling up some of her clothes.
As Facebook users shared accounts of the incident, they added spurious detail. One assailant became several. A groping became a rape. And a 17-year-old victim became an 11-year-old.
The police posted a statement reconstructing the rumor’s spread. But Mr. Guske knew a sober fact-check would never rise as high as a salacious rumor on Facebook’s News Feed, which promotes content based in part on its ability to keep users engaged.
So his team showed up at the homes of users who had first spread the rumor, showing them evidence that they had gotten it wrong. All but one removed or corrected their posts.
Karolin Schwarz, who runs a Berlin-based organization that tracks social media misinformation, said she often worked with German police departments to combat misinformation on social media. Mr. Guske’s team in Traunstein is, she said, “by the far the best I’ve seen.”
“Police departments should do this more,” Ms. Schwarz said. “It’s kind of great.”
Most law enforcement officials, she said, take the view that “it’s only the internet.” Even those who take the threat seriously rarely act quickly enough to stop a rumor’s spread.
“These things, if they reach thousands of people, you cannot get it back,” Ms. Schwarz said.
Mr. Guske lamented that Facebook, a $500 billion company, left it to overworked police departments to manage the risks created by its platform.
Last fall, a police chief in the southern Indian town of Gadwal drew attention for taking on an approach much like Mr. Guske’s to fight social media rumors. Countries in Asia and Africa, which represent the future of Facebook’s business, have proven most susceptible to violence linked to the platform, experts say.
Yet few police departments there have the resources of one in a German resort town like Traunstein.
Other police departments in Germany, where skepticism of Facebook runs deep, are taking a harder look at social media.
Gerhard Pauli, a state prosecutor in Hagen, a small city in the country’s postindustrial northwest, said his department was seeing more cases of violence that seemed to trace back to Facebook and other platforms.
When Andreas Hollstein, the mayor of neighboring Altena, was stabbed by an angry resident, the police concluded that social media outrage over the mayor’s pro-refugee policies had helped provoke the attacker.
“This is a danger we’ve always had in the history of humanity,” he said. “But nowadays, the technical capabilities are exploding.”
Though Facebook has worked with Germany’s regulation-prone government, Mr. Pauli said the German police still struggled to get the company’s attention.
Asked if he coordinated with Facebook, Mr. Guske said: “No, not really. That’s hard. It’s a problem.”
Mr. Guske is not an obvious sleuth for the digital age. He is a gray-haired police veteran, his 29 years of service mostly spent as a police inspector and, later, a liaison with local youth.
He settled into communications work, thinking that it would be quieter. Then the Group of 7 summit came to Bavaria in 2015.
Protesters came as well, and Mr. Guske’s role overseeing department social media brought him into contact with online rumors and posts whipping up outrage. Though social media was broadly considered frivolous at the time, Mr. Guske worried about its impact.
The next year, when Facebook rumors seemed to rise in concert with attacks on refugees, he retooled his team to track and try to halt the rumors. Though some dismissed the link as a coincidence, research released last year strongly suggests that online speech contributed to the violence.
Every rumor, Mr. Guske believes, undermines the credibility of officers like him who try to debunk it.
And because he is running his own little experiment, he can combat only the vitriol that originates in his jurisdiction.
Social media allows anyone, anywhere to spread rumors about his corner of the world. A number of white nationalists, taking a special interest in Bavaria’s culture wars, have done just that. And Mr. Guske is powerless to stop them.
“It’s hard to prevent fake news, because once Facebook pushes it —” he trailed off, shaking his head and slapping his palm on the table. “What more can you do?”