Russia’s effort to discourage minorities from voting for Hillary Clinton — described in a federal indictment released Friday — revives the mystery of why turnout among black voters declined so substantially from 2012 to 2016.

The indictment of 13 Russian trolls lays out a surprisingly sophisticated social media campaign to turn minorities, and black people in particular, against Clinton. Details of the campaign show that Russians understood both the significance of those voters in the election and the language of black social media.

“Russians understood how important minority voters were to Hillary Clinton’s chances in this election. They were able to read the situation and say, 'If we demobilize this community, it could have enough of an impact,'” said Bernard Fraga, an Indiana University election expert who studied black turnout in 2016.

Fraga’s research found that black turnout dropped most substantially in Michigan and Wisconsin — two states that Clinton narrowly lost and had expected to win.

The overall percentage of black Americans of voting age who cast a vote in 2016 fell to 59.6% in 2016 from 66.6% in 2012, the first decline in 16 years. Many experts say the decreased turnout severely hurt Clinton, who won 89% of the black vote, according to exit polls.

The indictment by special counsel Robert Mueller does not say that any Russian effort influenced the election results.

But the indictment does reveal that in the final months of the 2016 election campaign, Russian internet trolls “began to encourage U.S. minority groups not to vote” or to vote for a third-party candidate such as Jill Stein of the Green Party. Stein’s 51,500 votes in Michigan exceeded the 10,700-vote differential between Clinton and Trump. Her 31,000 votes in Wisconsin exceeded the 22,000-vote differential between Clinton and Trump.

President Donald Trump publicly thanked black voters a month after Election Day, saying they were “great to us” and that black people who did not vote were “almost as good” as those who voted for him.

The Russian effort to weaken Clinton’s support among black voters as spelled out in the indictment appeared sophisticated. Russian trolls used terms such as “Killary” — a Twitter hashtag created to insult Clinton — and formed an Instagram account called “Woke Blacks.”

Another Russian Instagram account, “Blacktivist,” bought an ad on Nov. 3, 2016, stating: “Choose peace and vote for Jill Stein. Trust me, it’s not a wasted vote.”

“This was pretty sophisticated in terms of using language that’s current, like the term ‘woke,'” said Christopher Anders, deputy director of the Washington legislative office of the American Civil Liberties Union.