WASHINGTON — Twelve Russian intelligence officers hacked into the Democratic National Committee and the Clinton presidential campaign in the run-up to the 2016 election and released tens of thousands of stolen communications in a brazen effort by a foreign government to meddle in U.S. politics, according to a grand jury indictment announced Friday.

The indictment stands as the clearest Justice Department allegation yet of Russian efforts to interfere, through illegal hacking, in the U.S. presidential election before Americans went to the polls — and the first to implicate the Russian government directly. It had been sought by special counsel Robert Mueller and comes days before President Donald Trump holds a summit with Russian President Vladimir Putin.

U.S. intelligence agencies have said the meddling was aimed at helping the Trump campaign and harming the election bid of his Democratic opponent, Hillary Clinton. The effort also included bogus Facebook ads and social media postings that prosecutors say were aimed at influencing public opinion and sowing discord on hot-button social issues.

The indictment lays out a sweeping effort starting in March 2016 to break into key Democratic email accounts, such as those belonging to the Democratic National Committee, the Clinton campaign and the Democratic Congressional Campaign Committee. Among those targeted was John Podesta, the Clinton campaign chairman.

The Kremlin denied anew that it tried to sway the election. “The Russian state has never interfered and has no intention of interfering in the U.S. elections,” President Vladimir Putin’s foreign affairs adviser, Yuri Ushakov, said Friday.

But the indictment identifies the defendants as officers with Russia’s Main Intelligence Directorate of the General Staff, also known as GRU. It accuses them of covertly monitoring the computers of dozens of Democratic officials and volunteers, implanting malicious computer code known as malware and using spearphishing emails to gain control of the accounts of people associated with the Clinton campaign.

By June 2016, the defendants began planning the release of tens of thousands of stolen emails and documents, the indictment alleges. The messages were released through fictitious personas like DCLeaks and Guccifer 2.0.

The charges come as Mueller continues to investigate potential coordination between Russia and the Trump campaign to influence the presidential election. The indictment does not allege that Trump campaign associates were involved in the hacking efforts or that any American was knowingly in contact with Russian intelligence officers.

The indictment also does not allege that any vote tallies were altered by hacking.

Still, Deputy Attorney General Rod Rosenstein said the internet “allows foreign adversaries to attack Americans in new and unexpected ways. Free and fair elections are hard-fought and contentious and there will always be adversaries who work to exacerbate domestic differences and try to confuse, divide and conquer us.”

Before Friday, 20 people and three companies had been charged in the Mueller investigation. The 20 are four former Trump campaign and White House aides, three of whom have pleaded guilty to different crimes and agreed to cooperate, and 13 Russians accused of participating in a hidden but powerful social media campaign to sway U.S. public opinion in the 2016 election.

If the involvement of the GRU officers in the hacking effort is proved, it would shatter the Kremlin denials of the Russian state’s involvement in the U.S. elections.

The GRU, which answers to the Russian military’s General Staff, is part of the state machine and its involvement would indicate that the orders to interfere in the U.S. election came from the very top.

Hours before the Justice Department announcement, Trump complained anew that the special counsel’s investigation is complicating his efforts to forge a better working relationship with Russia. Trump and Putin are scheduled to hold talks Monday in Finland, a meeting largely sought by Trump.

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.”

Referring to Mueller’s probe, he said: “We do have a — a political problem where — you know in the United States we have this stupidity going on. Pure stupidity. But it makes it very hard to do something with Russia. Anything you do, it’s always going to be, ‘Oh, Russia, he loves Russia.’”

“I love the United States,” Trump continued. “But I love getting along with Russia and China and other countries.”