WASHINGTON ― Last June, John Mattes started noticing something coursing like a virus through the Facebook page he helped administer for Bernie Sanders fans in San Diego. People with no apparent ties to California were friending the page and sharing links from unfamiliar sites full of anti-Hillary Clinton propaganda.

The stories they posted weren’t the normal complaints he was used to seeing as the Vermont senator and the former secretary of state fought out the Democratic presidential primary. These stories alleged that Clinton had murdered her political opponents and used body doubles.

Mattes, 66, had been a television reporter and Senate investigator in previous lives. He put his expertise in unmasking fraudsters to work. At first, he suspected that the sites were created by the old Clinton haters from the ‘90s ― what Hillary Clinton had dubbed “the vast right-wing conspiracy.”

But when Mattes started tracking down the sites’ domain registrations, the trail led to Macedonia and Albania. In mid-September, he emailed a few of his private investigator friends with a list of the sites. “Very creepy and i do not think Koch brothers,” he wrote.

Mattes and his friends didn’t know what to make of his findings. He couldn’t get his mind around the possibility that trolls overseas might be trying to sway a bunch of Southern Californians who supported Sanders’ run for president. “I may be a dark cynic and I may have been an investigative reporter for a long time, but this was too dark ― and too unbelievable and most upsetting,” he said. “What was I to do with this?”

By late October, Mattes said he’d traced 40 percent of the domain registrations for the fake news sites he saw popping up on pro-Sanders pages back to Eastern Europe. Others appeared to be based in Panama and the U.S., or were untraceable. He wondered, “Am I the only person that sees all this crap floating through these Bernie pages?”

He wasn’t. Bernie supporters across the country had been noticing dubious websites and posters linked back to Eastern Europe long before Mattes did ― and even before The Washington Post reported in mid-June that Russian government hackers had stolen emails from the Democratic National Committee. They had been warning each other that something weird was going on, posting troll alerts and compiling lists of fake news sites.

There is enough real news to fight over, they thought, without arguing over anti-Hillary conspiracy theories from Macedonia.

Stephen Lam/Reuters Bernie Sanders' supporters started seeing fake news in early 2016.

Sometimes it was hard to tell who was doing the trolling and for what purposes. Aleta Pearce, 54, who lives in Malibu, California, was an administrator of half a dozen pro-Sanders Facebook groups and a member of many others. In May 2016, she posted a memo to various Facebook groups about the fake news issue, warning of bogus sites.

“The pattern I’m seeing is if a member is repeatedly posting articles that are only from one URL that person is just there to push advertising,” Pearce wrote. “They probably have a sock account with little to no content. They are often from Russia or Macedonia.” (A “sock” or “sock puppet” account uses a false identity to deceive.)

Pearce added, “Please share this with other Bernie groups so we can put an end to this spam bombing that’s filling up our pages and groups. It’s time to chase the mice out of the hen house and send them a message. They don’t know who they are messing with.”

The first tidal wave of spam was mostly anti-Bernie, Pearce recalled, posted by Clinton backers. (David Brock’s Clinton-backing super PAC had likely paid for some portion of those.) But after Clinton became the Democratic nominee in July, Pearce noticed a switch to anti-Hillary messages with links to fake news and to real news with obnoxious pop-up ads.

“Every site publishing those ― you clicked on the article, you would be slammed with ads and strange articles,” Pearce told HuffPost. “It was overwhelming. It was 24/7.”

She kept a list of fake news sites to watch for ― it grew into dozens. There were posts on the Clinton-has-Parkinson’s conspiracy and the Clinton-is-running-a-pedophilia-ring-out-of-a-pizza-shop conspiracy.

On the Sanders campaign, it was Hector Sigala’s job to connect with all the organic Facebook groups. He recalled seeing “a lot of trolls” try to convince people of something “that was obviously fake.”

Many of the interlopers, Sigala said, claimed to be Sanders fans who had decided to vote for GOP nominee Donald Trump or Green Party candidate Jill Stein in the general election and tried to convince others to do likewise. “It made it seem like the community as a whole was supporting that, but that wasn’t the case,” he said.

Sigala thinks most of them were just your average internet trolls. He said he found many were members of 4chan, a gathering place for the alt-right, white nationalists and plain old nihilists from which has sprung all manner of mischief.

The Sanders campaign had begun seeing this particular brand of fake news starting in early 2016. “The first time that we kind of fell for it, for like two minutes, was this link from what seemed to be ABC News,” Sigala said. It turned out to be ABC.com.co, a fake site that has no affiliation with the real news network. It had “reported” that the pope himself had endorsed Sanders.

It came in like a wave, like a tsunami. It was like a flood of misinformation. Bev Cowling, who administered Facebook groups for Sanders supporters

In trying to wade through the flood of fake news, Sanders supporters had some serious trust issues. There was good reason to be skeptical of Clinton and the WikiLeaks dump of DNC emails was real, after all. But a steady diet of stories fabricated out of thin air can also feed into paranoia and flame wars.

Bev Cowling, 64, saw a sudden deluge of requests to join the Sanders Facebook groups she administered from her home in Toney, Alabama. All of a sudden, they were getting 80 to 100 requests to join each day. She and the other administrators couldn’t vet everyone, and the posts started getting bizarre. “It came in like a wave, like a tsunami,” she said. “It was like a flood of misinformation.”

Cowling, a retired postal worker, said some of her Facebook group members were ready to believe the bogus news links. “People were so anti-Hillary that no matter what you said, they were willing to share it and spread it,” she said. “At first I would just laugh about it. I would say, ‘C’mon, this is beyond ridiculous.’ I created a word called ‘ridiculosity.’ I would say, ‘This reeks of ridiculosity.’”

But Cowling got pushback. She was called a “Hillbot” and a Trump supporter. She ended up removing dozens of members who refused to stop pushing conspiracy theories. “I lost quite a few friends,” she said.

Matthew Smollon, a 34-year-old copy editor and page designer based in Knoxville, Tennessee, noticed an influx of posts linking to fake news as early as January 2016. So much of it, Smollon noticed, came from the same accounts. Almost all the sites he traced went back to Veles, Macedonia, which Wired magazine has since dubbed the “Fake News Factory to the World.” There wasn’t a single link he found that went to a pro-Clinton fake news story.

None of the fake stories stood out to Smollon. He described the Facebook groups as “being in a room filled with blasting televisions.” It was hard to pick out the loudest noises. “The ultimate goal of this wasn’t so much misinformation as distraction from valid info,” he concluded.

But Smollon had a hard time convincing other Bernie supporters that they were being played. “No one cared,” Smollon said. “At that point, you were a Hillary shill. It was like an echo chamber of anger.”

Even when pointing out that something like NBCPolitics.org was a fake site ― the real site is NBCNews.com/politics ― he drew criticism. He was eventually removed as a moderator from one of the pro-Sanders Facebook groups. “It’s the closest I’ve been to being gaslit in my life,” he said.

In June, Smollon posted a piece on Medium with the headline, “Dear Bernie Supporters: Stop sharing posts from dumpster fire websites.” He urged his fellow Sanders fans to wake up:

Guys, I sincerely love you. I love your passion. I love your fire. I love all of that. But when 400 people are circle-jerking clickbait links in between wondering how Hillary Clinton is behind the FEMA Earthquake drill that happens on several days with one of them being primary day? Holy shit. You are allowing yourselves to be manipulated. Through the practice of taking anything that agrees with your opinion at face value, actively refusing to believe anything but what agrees with your narrative and following that up with blatant disregard for doing two minutes of searching to verify the information: you become the myopic Trump supporter that you so vocally loathe.

Some people “liked” his Medium piece on Facebook and posted it on their walls, he said. Others did not. Smollon later updated his article to say he’d been banned from the group “Bernie Believers” because of it.

“This is a pretty solid case for admins/mods being part of the spam,” he wrote. “Not all of them obviously, but it only takes one person running with an ulterior motive to ensure the whole thing goes to shit.”

Screenshot A Facebook user named Oliver Mitov posted dubious news links about Hillary Clinton.

In San Diego, Mattes was intrigued by a Facebook user named “Oliver Mitov” whom he saw constantly posting anti-Clinton propaganda.

Mattes first noticed Mitov posting in his Facebook group in September. But when he searched the page’s archives, he found that Mitov had been in the group since late July. He soon realized there wasn’t just one Mitov but four. Three had Sanders as their profile picture. Two had the same single Facebook friend, while a third had no Facebook friends. The fourth appeared to be a middle-aged man with 19 Facebook friends, including that one friend the other Mitovs had in common.

All combined, the four Mitovs had joined more than two dozen pro-Sanders groups around the U.S., including Latinos for Bernie Sanders, Oregon for Bernie Sanders 2016 and Pennsylvania Progressives for Bernie Sanders. Together, those groups had hundreds of thousands of members.

The Mitov posts would have been explosive if they’d been true. In one Aug. 4 post to Mattes’ page, Mitov wrote, “This is a story you won’t see on Fox/CNN or the other Mainstream media!” He then linked to a post claiming falsely that Clinton had “made a small fortune by arming ISIS.” On Sept. 25, he posted on several pro-Sanders pages a link promising game-changing information: “NEW LEAK: Here is Who Ordered Hillary To Leave The 4 Men In Benghazi!” The link went to a fake news site called usapoliticsnow.com.

The aim of Mitov’s activity seemed pretty obvious to Mattes: to depress the number of Sanders supporters who voted for Clinton in November.

“He was a ringer,” Mattes said.

Mattes tried to friend the various Mitovs and message them. None of them responded, he said. Attempts by HuffPost to reach Mitov were similarly unsuccessful.

Screenshot Mitov's long list of pro-Sanders Facebook pages shows no other outside interests.

Keegan Goudiss, who ran digital advertising for Sanders’ presidential bid, had a different perspective on the trolling. He launched paid campaigns on social media and around the internet, so he was very familiar with the way that money can drive a meme.

Bots and trolls that spread fake news shouldn’t be ignored, he said, but “it’s like pissing in the ocean. There’s a lot of noise online.” One way to help your message cut through the noise is to spend money with Facebook, Google or an ad targeting platform that spreads links all over the internet, often at the bottom of stories. (Scroll down far enough on this page and you’ll probably see some of them.)

Goudiss recalled one telling example of how this worked: A Clinton ad appeared in the middle of a row of links, clearly paid for by a pro-Clinton group targeting potential donors and voters. To its left was a story making bogus claims about an illegitimate Clinton child. To its right was a piece on presidential mistresses. “There seems to have been a concerted effort to tarnish Hillary and people in her campaign’s reputation using paid placement,” he said.

Screenshot This screenshot captured by Keegan Goudiss during the campaign shows fake news to the left, Clinton fundraising in the middle and an anti-Clinton story to the right.

He can’t prove who was doing that, Goudiss said, but it’s probably worth trying to figure out.

“Was there a Russian entity supporting those websites that popped up?” he said. “That’s important and people deserve to know who influences our democracy.”

Some level of foreign participation in spreading disinformation about the left was comically apparent. The names of a few suspect Facebook groups reek of poor translation. One group with more than 80,000 members, claiming to be from Burlington, Vermont, is called “Bernie Sanders Lovers” ― the kind of name a non-English speaker might think makes sense, but that sounds wrong to native ears.

Screenshot

Throughout the campaign, the Bernie Sanders Lovers page saw heavy engagement, and nearly every article it shared was from Bients.com, the pieces posted there by one Maximilian Gottlieb. Gottlieb, in turn, pulled articles from other sources, some more and some less reliable.

On Oct. 29, for instance, he put up 11 articles. A few praised Trump or gave Trump’s advisers space to attack Clinton. Others attacked Clinton’s campaign directly: adviser Huma Abedin had ties to radical Islam (false), the DNC email leak was authentic (true), campaign manager Robby Mook had deleted his entire Twitter feed (false).

Since the election, the Bernie Sanders Lovers page has shifted to urging the senator to run for president again in 2020. It no longer shares Bients.com stories. Instead, they all come from ThePredicted.com. Both sites were registered by a person name Hysen Alimi in Albania. (Feel free to check out the sites yourself, but Chrome will warn you your connection is “not secure,” so don’t enter any information there.)

Screenshot A day of posts from Maximilian Gottlieb.

There had been rumblings that the Russians were specifically behind the DNC hack since last June. In early October, the U.S. intelligence community said it was “confident” that President Vladimir Putin’s government had both directed the hack and made sure the emails found their way to WikiLeaks. In January of this year, a more detailed intelligence report concluded that the Russian government had blended covert intelligence operations with overt efforts by, among others, “paid social media users or ‘trolls’” to try to influence the U.S. election.

A separate dossier on Russia’s role, assembled by former MI6 agent Christopher Steele and made public by BuzzFeed, claimed that the DNC leak had been an attempt to “swing supporters of Bernie Sanders away from Hillary Clinton and across to Trump.”

“These voters were perceived as activists and anti-status quo and anti-establishment and in that regard sharing many features with the Trump campaign, including a visceral dislike of Clinton,” Steele wrote.

The intelligence report also said that the DNC hackers seemed to have financial ties to the Internet Research Agency, a Saint Petersburg, Russia, company that has taken state-sponsored trolling to an industrial level. Its likely financier is “a close Putin ally with ties to Russian intelligence,” the report stated.

“Russia’s information war might be thought of as the biggest trolling operation in history,” wrote The New York Times in a 2015 profile of the firm, “and its target is nothing less than the utility of the Internet as a democratic space.”

The “Internet as a democratic space” is the very thing that allowed the energy of the Sanders campaign to snowball into a movement for change. It was also the thing that allowed Oliver Mitov and his ilk to thrive.

Could the fake news tsunami have swung the election? It’s impossible to say for sure, but a YouGov survey recently asked people who voted for Sanders in the primary how they thought other people they knew who backed Sanders ended up voting in the general election. Thirteen percent said all of those folks voted for Clinton, and 48 percent said most of them did. But 20 percent said only some, 9 percent said just a few and 4 percent said none voted for Clinton.

In the survey, only 7 percent said that most or all of the Bernie people they knew wound up helping raise money or otherwise volunteering for Clinton. Fifty-four percent said that applied to just a few or none of the Bernie people they knew. Sanders backers were by far the most energized element of the Democratic coalition during last year’s campaign. Clinton’s inability to motivate them more broadly to back her candidacy undoubtedly hurt.

Asked what they themselves did, 12 percent of those who voted for Sanders said they went on to volunteer or raise money for Clinton. Only 16 percent of those who voted for Clinton in the primary said they also volunteered for her.

Of course, the propaganda didn’t create the chasm dividing left-wing voters. The belief that the DNC favored Clinton was widely held. Fifty-eight percent of all the survey respondents agreed on that. Among Democrats, the number was 55 percent. In the Midwest, which essentially elected Trump president, 67 percent agreed. Even 62 percent of those who voted for Clinton in the primary said that the DNC favored her.

But the legitimate skepticism opened the door to believing the more demented propaganda. And the more the fake news was passed around, the harder the divisions became. Clinton backers would charge Sanders supporters with being obnoxious, sexist “Bernie bros.” Many of those bros may have been trolls, not real Sanders supporters. Tell that to a Clinton backer, however, and you can be accused of dismissing the hostility they faced.

Aidan King set up a popular Reddit page for Sanders beginning in 2013 and went to work for the campaign in January 2016 as Sigala’s deputy. He dealt directly with many of the Facebook groups. After the Democratic convention, he said he noticed a strong shift away from the party in the tone of many of those pages.

“I’ve gone back and forth on it,” King said. “I wouldn’t feel comfortable saying with any authority it’s a coordinated effort by trolls, but also wouldn’t feel confident saying it was exclusively pissed-off Bernie supporters.”

It might not actually matter if Vladimir Putin or a kid in Macedonia masterminded the flood of fake news. What matters is that it happened ― and it is still happening. People are deliberately seeding misinformation into the left-wing conversation. That’s a real fact. (Trying to measure the size and scope of the operation could make for a useful political science dissertation.)

For a wide swath of Sanders backers, the primary is still far too raw to even start to think about Russia. Mentioning foreign sabotage sounds like you’re throwing up a smokescreen to obscure the Democratic establishment’s own failure. But Mattes has tried to argue that two things can be true at once: Clinton was a terrible candidate and Russia intervened in the U.S. election.

“It’s wildly distressing that we were played,” Mattes said.

UPDATE: March 13 ― The Facebook page “Bernie Sanders Lovers” responded to this article on Sunday, saying: “We were never linked up with Russians and we will never be with them.”

“How come that we would write of Trump’s advantage and support him when we are democrats,” the page administrator wrote. “Even though we are democrats we do not support Hillary Clinton. We are not linked with the government to support someone that we do not like, to be encouraged to support someone, but we are those who desire and want progress.”

After reading this story, we’re curious what your view is on Russia’s role in the election. Take this brief survey, and we’ll post the results here on Sunday night.

CORRECTION: Keegan Goudiss ran digital advertising, not all digital efforts, for Bernie Sanders’ presidential bid.