The first indications of the subject matter came shortly before the conference began with reports that a notice posted in the main federal courthouse in Washington, DC, indicated that attorneys from Mueller's Special Counsel's Office and the DOJ National Security Division had visited Friday morning related to a grand jury indictment.
Rosenstein said that while the defendants corresponded with several Americans through the internet, there was "no allegation in the indictment that the Americans knew they were communicating with Russian intelligence officers".
Justice Department special counsel Robert Mueller is continuing to investigate whether anyone in the United States conspired with the Russian attack on the election.
The announcement comes as Trump prepares to meet Russian President Vladimir Putin at a summit in Helsinki next week.
Thousands of emails taken from the accounts of staff at the the Democratic National Committee (DNC) and John Podesta, Hillary Clinton's campaign chairman, were published by outlets including WikiLeaks during the 2016 campaign. They also hacked into the computer network of the Democratic National Committee.
On Feb. 16, his prosecutors charged 13 Russians and three Russian entities they said were part of a broader effort to sow discord among USA voters through social media - which they used to impersonate Americans, coordinate with unwitting US activists and even plan rallies. But that indictment had not directly tied the meddling effort to Russia's government.
A few hours before the indictments were announced, Trump described the Mueller investigation as a "rigged witch hunt" that is hurting the US relationship with Russian Federation.
Trump said at a news conference Friday near London with British Prime Minister Theresa May that he wasn't going into the meeting with Putin with "high expectations".