Special counsel Robert Mueller has filed conspiracy and fraud charges against 13 Russian nationals and three organizations. The group is charged with defrauding the United States as part of a campaign to influence the US election.

Ten of the defendants were allegedly employed by the Internet Research Agency, a “troll farm” funded by the Russian government for disinformation efforts. “Defendants, posing as US persons and creating false US personas, operated social media pages and groups designed to attract US audiences,” the indictment reads. “They engaged in operations primarily intended to communicate derogatory information about Hillary Clinton, to denigrate other candidates such as Ted Cruz and Marco Rubio, and to support Bernie Sanders and then-candidate Donald Trump.”

Prosecutors seem to have had access to internal communications within the Internet Research Agency. In one instance on September 14th, an IRA employee managing the “Secured Borders” Facebook group was criticized for having too few posts criticizing Hillary Clinton. According to the indictment, a supervisor told him “it is imperative to intensify criticizing Hillary Clinton” in future posts.

In another incident in August, the group wired money to the United States, instructing a contractor to “build a cage large enough to hold an actress depicting Clinton in a prison uniform.” Most of the group’s activities were routed through a US-based VPN service, so the traffic would appear to be originating within US borders.

Not all of the activities were pro-Trump. After the election, the group allegedly used many of its false accounts to “organize and coordinate US political rallies in support of president-elect Trump, while simultaneously using other false US Personas to organize and coordinate US political rallies protesting the results of the 2016 presidential election.”

“This indictment serves as a reminder that someone isn’t always who they say they are on the internet,” said deputy attorney general Rod Rosenstein in a press conference announcing the charges.

Social networks have struggled to come to grips with Russian disinformation efforts in the months since the 2016 election, despite demands from the US Congress and UK parliament. Recent estimates suggest roughly 126 million Facebook users were exposed to Russian disinformation accounts during the 2016 campaign, and Twitter has notified at least 1.4 million users they may have been exposed. It’s unclear how heavily today’s indictment draws on information provided by Facebook and Twitter, although the companies would be the richest source of information on how the campaigns spread.

In a statement, Joel Kaplan, Facebook’s vice president of global policy, said, “Today’s news confirms our announcement last year that foreign actors conducted a coordinated and sustained effort to attack our democracy. As we said publicly last year, this kind of foreign interference violates all of our values. These indictments now say it also violated the law.” Kaplan says the company “proactively disclosed IRA activity” to Mueller, Congress, and the public.

“We know we have more to do to prevent against future attacks. We’re making significant investments, including increasing the number of people working on security from 10,000 to 20,000 this year. We’re also continuing to work closely with the FBI, the Department of Homeland Security and other companies on better ways to protect our country and the people on our platform,” Kaplan says. “We’re particularly encouraged by the FBI’s creation of a task force dedicated to addressing election interference, and we are actively working with them. We’re committed to staying ahead of this kind of deceptive and malevolent activity going forward.”

Twitter, which originally declined to comment on the news, sent out a statement later today saying, “Russian efforts to disrupt the 2016 US election, in part by abusing social media platforms, go against everything we at Twitter believe. Any activity of this kind is intolerable, and we all must do more to prevent it.” The company says it also continues to work with the Mueller investigation and with Congress.

“We are also continuing to be transparent with the public as our own review of the 2016 election has evolved. We look forward to working with the FBI Task Force to assist companies and the public in identifying foreign manipulation efforts through social media platforms,” the statement reads. “As we said last fall, tech companies cannot defeat this novel, shared threat alone. The best approach is to share information and ideas to increase our collective knowledge, with the full weight of government and law enforcement leading the charge against threats to our democracy.”

The company says it’s “committed to providing a service that fosters and facilitates free and open democratic debate and that promotes positive change in the world,” and also that “we are committed to addressing, mitigating, and ultimately preventing any future attempts to interfere in elections and the democratic process, and to doing so in the most transparent way possible.”

Update at 9:22PM ET, 2/16: Added statements from Facebook and Twitter.