Special counsel Robert Mueller on Friday indicted 13 Russians, and three Russian organizations, with meddling in the 2016 US election.

The charges centered on the alleged use of social media to sow political discord, via inserting politically charged messages into the stream of American electronic discourse. But they dealt with physical actions as well: Russians attempted to organize rallies in the US, and at one point paid for a demonstration that included a fake prison cage housing someone portraying Hillary Clinton.

Underlying the allegations was this stark message: A foreign power is attempting to take advantage of America’s stark, widening political and demographic divisions to undermine the nation’s great strength of free expression and debate.

Also, the surprise nature of the indictment emphasized the difficulty, and perhaps the futility, of predicting where Mr. Mueller’s investigation might lead. He and his investigators obviously have vast amounts of information that aren’t public. There have been no leaks attributable to his organization. There will be more such big news surprises to come.

“The special counsel’s investigation is ongoing,” Deputy Attorney General Rod Rosenstein said in announcing the indictment.

In sum, the indictment charges the Internet Research Agency, a group operating out of St. Petersburg, with paying for social media ads and organizing fake social media posts that dealt with the US election since 2014.

By early to mid-2016, the group decided that it would focus on defeating Hillary Clinton, by supporting Donald Trump and Bernie Sanders in upcoming votes, according to the indictment.

The Russian operation was dubbed the “translator project” and had a budget of about $1.2 million per month. It dealt directly with many Americans, but those Americans didn’t know who they were dealing with, according to the Justice Department. All cooperation, including some cooperation from Trump campaign officials, was “unwitting,” according to the indictment.

Specialists, divided into teams that worked day and night shifts, produced hundreds of media posts, such as “Ohio wants Hillary 4 prison!” and “Among all the candidates only Donald Trump is the one and only one who can defend the police from terrorists,” said the Justice Department.

The indictment is likely to make it much more difficult for President Trump to downplay the Russia investigation as a “witch hunt” or “Democrat hoax,” as he has in the past. The Justice Department allegations are highly detailed and appear backed up by copious evidence, including document and electronic media trails.

Indeed, the White House did not dismiss the account. A statement said that Mr. Trump had been briefed on the findings and that the indictment proved “that there was NO COLLUSION between the Trump campaign and Russia and that the outcome of the election was not changed or affected.”

In announcing the indictment, Deputy Attorney General Rosenstein had said that all the Americans named in the document had been unwitting participants, and that it contained no evidence that any of the social media posts had thrown the 2016 vote.

But Rosenstein’s comment was pointedly limited to today’s indictment. Given today’s events, what else does Mueller have? It was difficult to not read a bit into the press conference, and see it as the special counsel and the Justice Department quietly demonstrating that they will continue to go about their jobs despite Trump tweets questioning their competence.