Deputy Attorney General Rod Rosenstein told reporters Friday that the defendants worked for the GRU, Russia's premier military intelligence agency.
The 12 alleged Russian intelligence officials were indicted as part of the ongoing investigation into Russia's interference in USA politics, Rod Rosenstein, the deputy attorney general, was due to announce at a press conference in Washington.
The Russians were accused of hacking into the computer networks of the Democratic National Committee and the presidential campaign of Hillary Clinton.
Additionally, former National Security Adviser Michael Flynn and Trump campaign adviser George Papadopoulos have pleaded guilty to making false statements to the Federal Bureau of Investigation about the contacts they had during the campaign with Russians.
In addition to the DNC hack, the defendants are charged with a separate conspiracy to "to infiltrate computers of organizations responsible for administering elections, including state boards of election, secretaries of state, and companies that supply software and other technology used to administer elections".
Of the Russian interference - which Trump has repeatedly expressed doubts about - Tapper said, "According to his own justice department, it's not a joke".
The indictment - which comes days before President Donald Trump holds a summit with Russian President Vladimir Putin - was the clearest allegation yet of Russian efforts to meddle in American politics.
The charges come after Mueller's investigation has already led to the indictment of 13 Russian nationals earlier this year.
"It's important not to think as Republicans or Democrats, but patriotically as Americans", he said.
They corresponded with 'several Americans, ' he said, but the DOJ is not alleging htat any of them knew they were talking with Russian agents.
Mr Rosenstein said the defendants corresponded with several Americans during the alleged conspiracy, but added there is no allegation that any United States citizen committed a crime.