Special counsel Robert Mueller has indicted 13 individuals for their attempts to meddle in the 2016 election. Twelve of them worked for the Internet Research Agency, a notorious Kremlin-linked Russian troll farm.

The indictment says the Internet Research Agency, which is based in St. Petersburg, “engaged in operations to interfere with the elections and political processes” starting as early as 2014.

It did so by spewing divisive propaganda, posing as US activists and posting politically charged content on social media or through online ads and encouraging people to attend rallies in person. The group used this campaign to sow discord, and by mid-2016, had settled on the goal of supporting Donald Trump and “disparaging Hillary Clinton.”

Overall, the indictment reveals in startling detail the extent of the Internet Research Agency’s disinformation operations during the 2016 election. Here are four things to know about the shadowy Russian entity at the center of the latest Mueller indictment.

1) The Internet Research Agency spread false stories and propaganda — making it hard to tell what was true

In 2015, Adrien Chen, in New York Times Magazine, wrote what is probably the definitive account of the obscure Internet Research Agency. Employees of the troll farm worked out of a nondescript building in St. Petersburg, which had “industrialized the art of trolling.”

Not all of it was for a foreign audience — much of it included pro-Kremlin propaganda for a domestic audience around hot-button issues, such as Ukraine. But in 2014, a hacker group published leaked emails said to be from the Internet Research Agency that revealed an expanding English-speaking operation. The hack included a list of troll accounts — accounts that, in some cases, interacted with and were followed by other troll accounts:

One account was called “I Am Ass.” Ass had a Twitter account, an Instagram account, multiple Facebook accounts and his own website… Ass had a puerile sense of humor and only a rudimentary grasp of the English language.

Ass had a half-dozen or so fans who regularly liked and commented on his posts. These fans shared some unusual characteristics. Their Facebook accounts had all been created in the summer of 2014. They all appeared to be well-dressed young men and women who lived in large American cities, yet they seemed to have no real-life friends. Instead, they spent their free time leaving anti-Obama comments on the Facebook posts of American media outlets like CNN, Politico and Fox News… It became clear that the vast majority of Ass’s fans were not real people. They were also trolls.

These accounts began to spread disinformation in the United States, and Chen noted a few coordinated campaigns that linked back to the Internet Research Agency: a fake chemical explosion in Louisiana on the anniversary of 9/11 in 2014 — complete with a doctored CNN page passed around on Twitter and a real text message sent out to area residents — and a fake Ebola outbreak and hoax police shooting in Atlanta in December 2014.

Chen also attended a live event in New York City called “Material Evidence,” which Chen described as “a real-life version of the hall of mirrors I’d stumbled into on Facebook”: an exhibit devoted to bizarre Russian propaganda about Ukraine and Syria.

In Russia, “by working every day to spread Kremlin propaganda,” Chen wrote, “the paid trolls have made it impossible for the normal Internet user to separate truth from fiction.” The US-focused efforts suggest a similar goal.

2) The Internet Research Agency was behind Russian attempts to influence the election on social media

The Internet Research Agency operated accounts that tried to influence political opinion in the United States during the 2016 election. They seized on controversial issues and exploited racial and ideological fissures to inflame tensions. A sampling of the account pages included “Secured Borders” and “Blacktivist.”

Millions of people saw or interacted with such posts and accounts. Facebook testified that about 126 million Facebook users had seen Russian-linked content. In December, the social-media giant even rolled out a feature where users could figure out if they interacted with content created by the Internet Research Agency.

Twitter found more than 3,800 accounts linked to the Internet Research Agency and alerted about 1.4 million US users that they may have interacted with Kremlin propaganda. Those accounts, now shut down, posted 176,000 tweets in the 10 weeks before the election.

Google, too, found two accounts linked to the Internet Research Agency spent that $4,700 on ad platform. The platform also detected 1,000 videos from Russian-linked accounts ahead of the election.

One popular account, @TEN_GOP, amassed more 100,000 followers, according to the indictment. Not all of its content was political or inflammatory. But when it was, it could be explosive.

Their tweets were also retweeted and amplified by people who had far more followers — Michael Flynn, Michael Flynn, Roger Stone, Nicki Minaj, Ann Coulter and Chris Hayes. The account was finally shut down in August, though the real Tennessee GOP had spent monthstrying to get it shut down.

“They were trying to influence influencers,” Jonathan Albright, research director at the Tow Center for Digital Journalism at Columbia University told the Washington Post. “They’re creating buzz.”

3) The professional trolls worked hard to seem American

According to the indictment, Russian trolls divided up their shifts to make sure they’d be able to post in different US time zones. They also received “guidance” on US holidays so they could post accordingly — a sort of virtual disguise.

The Moscow Times reported that workers were told to watch “House of Cards” and given grammar lessons.

Journalists with Russian newspaper RBC reported that about 90 staffers were assigned to the “US desk” this fall; they also reported that the Internet Research Agency’s trolls tried to recruit about 100 US activists to help coordinate their political activities. The Mueller indictment also alleges that Russian used fake personas to reach out to “over 100 real U.S. persons” as part of their scheme.

But posting Russian propaganda also sounds a lot like any other job. They had quotas they needed to meet; they got paid. (A salary, according to Chen, that was “surprisingly high for the work.”)

Vitaly Bespalov, 26, who told NBC News he used to work for the Internet Research Agency likened it to a normal IT firm that instead trafficked “a merry-go-round of lies.”

Again, the US operations were just a small part of the larger propaganda machine. As NBC News reported:

Writers were separated by floor, with those on the third level blogging to undermine Ukraine and promote Russia. Writers on the first floor — often former professional journalists like Bespalov — created news articles that referred to blog posts written on the third floor. Workers on the third and fourth floor posted comments on the stories and other sites under fake identities, pretending they were from Ukraine. And the marketing team on the second floor weaved all of this misinformation into social media.

Bespalov said they would create accounts with names and locations, pump out content until those accounts were blocked, and then start fresh.

4) The Kremlin connection is a figure known as “Putin’s cook”

The Internet Research Agency is usually described as “Russian government-linked” agency. US intelligence agencies have concluded that the effort to interfere in the US elections was linked to the “highest levels” of the Russian government, and it seems clear that the activities of the Internet Research Agency fit squarely into that larger operation.

The indictments make the nature of that link a little clearer: A Putin ally, Yevgeny Prigozhin was indicted Friday for allegedly financing the Internet Research Agency through his companies, specifically Concord Management, and other subsidiaries, according to the Justice Department.

The New York Times described Prigozhin, who’s known as “Putin’s cook,” as the Russian leader’s “go-to oligarch for that and a variety of sensitive and often-unsavory missions.” He was sanctioned by the US in December 2016, and Chen’s 2015 piece had also made the connection that Prigozhin was the Internet Research Agency’s likely financier.

Not surprisingly, Russian President Vladimir Putin, through his spokesman, has denied any Kremlin connection to the troll farm.