The investigation by special counsel Robert Mueller into Russian interference in the 2016 election took a significant step forward Friday, with the indictment of 13 Russian nationals and and three Russian organizations along with the announcement of a guilty plea from a California man for identity fraud.

The indictment and guilty plea are important because they are the first charges related to Russian attempts to sabotage the election. (The other indictment and pleas in the Mueller investigation have been in relation to other criminal activities.) The election interference is an activity long denounced by the U.S. intelligence community but routinely called into question by President Trump.

The indictment is also the latest sign that Mueller’s investigation is still moving quickly and ambitiously in pressing charges against those involved.

Our analysis of special counsel probes in the modern era, starting in 1979, puts Mueller’s investigation in select company for producing criminal charges at all — a majority of the investigations over the past four decades ended without charges being filed against anyone.

The total number of individuals charged in the investigation is now up to 18, including an indictment and two guilty pleas from last fall. One of the defendants from last fall, Rick Gates, is reportedly finalizing a plea deal, which would signal that he’s ready to cooperate with the investigation.

Historically, major special counsel investigations that have led to charges have lasted for years, with indictments and guilty pleas trickling out as an inquiry gains momentum. So more charges seem likely to come.