There was a lot of testimony during this past week's impeachment inquiry about foreign interference in our 2016 election, including the president's assertion that Ukraine was involved. But the president's own intelligence agencies say it was the Russians who "hacked" the 2016 elections. Special counsel Robert Mueller spelled it out in his report.

Now the Justice Department has at least two open cases against Russian citizens for interfering with our presidential and congressional races, we decided to take a closer look at one of them - the case against 12 Russian military officers accused of breaking into the Democratic Party's computers, stealing compromising information, and selectively releasing it to undermine Democratic candidates. There's no evidence of similar operations against Republicans in 2016. With the 2020 election approaching, the story of "The Russian Hack."

Robert Anderson: The Russians never left. I can guarantee you in 2016 after this all hit the news, they never left. They didn't stop doing what they're doing.

Bill Whitaker: This wasn't just a one-time thing?

Robert Anderson: No way. Russia doesn't do it that way.

Robert Anderson

Robert Anderson should know. He spent 21 years inside the cloak and dagger world of spies and hackers overseeing the FBI's counterintelligence and cyber Divisions and tracking Moscow's spy agencies, an alphabet of artifice, the FSB, SVR, and, especially, the GRU.

Robert Anderson: The GRU is military intelligence. So when we look at the attacks that happened during our presidential races in 2016 you had military organizations inside of Russia attacking our infrastructure.

Bill Whitaker: So are they hackers or are they soldiers?

Robert Anderson: So they're both. And in most cases, in most of these units, they're not just hackers, they're probably some of the best mathematical minds in Russia. These are seasoned professionals that have worked their way up the ranks to be in these units to carry out these strategic attacks on behalf of that country.

The hacker-soldiers from GRU unit 26165

These are the hacker-soldiers from GRU unit 26165 who, according to the Justice Department, were responsible for "breaking and entering" into the Democratic Party's computers remotely, from Moscow. Their names, ranks and faces are now on the FBI's most wanted list for stealing, among other things, the Democrats' strategic plans, detailed targeting data, and internal polling. GRU Colonel Aleksandr Osadchuk commanded a separate unit, 74455. One of his officers was in charge of spreading the stolen material to political operatives, bloggers and the media. Another hacked state election boards.

Bill Whitaker: It wasn't some 400-pound guy in his parent's basement?

Robert Anderson: No. This was a well-choreographed military operation with units that not only were set up specifically to hack in to obtain information, but other units that were used for psychological warfare were weaponizing that. This is not an operation that was just put together haphazardly.

"It's not different than Watergate."

The Justice Department's National Security Division is overseeing the Russian hacking case.

Assistant Attorney General John Demers runs the division, along with deputies Adam Hickey and Sean Newell. DOJ attorney, Heather Alpino, worked with special counsel Mueller on the Russian indictments. All have access to the underlying intelligence, and have no doubt the Russians interfered in the 2016 election.

Bill Whitaker: This really happened.

John Demers: Yes. That really happened. And we believe that if we had to we could prove that in court tomorrow using only admissible, non-classified evidence to 12 jurors.

Bill Whitaker: Do you ever expect to get the 12 Russian officials to trial?

John Demers: I would be surprised. But the purpose of the indictment isn't just that, although that's certainly one of the purposes. The purpose of this kind of indictment is even to educate the public.

For a legal document, the 29-page indictment is a page-turner. It details how U.S. intelligence agencies tracked each defendant's actions, sometimes by the keystroke, revealing the fictitious names and phony emails used to infiltrate the Democrats' computers, and tracing the stolen data on its circuitous route from Washington, D.C. to Moscow.

John Demers

Bill Whitaker: The information in the indictment is very detailed. You have descriptions of the Russian agents typing into their computers.

John Demers: Obviously I can't go into too much detail because I don't wanna reveal investigative methods. But the insight here is that behind every one of those keyboards is not an IP address. It's a human being.

Those indicted GRU agents. The U.S. says one team, working out of a building in Moscow called the "Tower," created a website and a provocative character to disseminate the stolen material: Guccifer 2.0.

John Demers: So Guccifer 2.0 is a fictional online persona. It's all an effort on the Russian side to hide their involvement.

Bill Whitaker: And these guys are pretending to be one lone hacker.

John Demers: Correct.

Bill Whitaker: And that works?

John Demers: What it gives them is plausible deniability, right? They don't need for it to work 100% as long as the Russians can say: "Wasn't us."

Posing as Guccifer 2.0, the Russians offered up stolen documents to Julian Assange's WikiLeaks and self proclaimed "dirty trickster" Roger Stone. It was all part of a broad campaign to disrupt the presidential election. But there was another, less well-known part of the Russian operation: to undermine Democrats running for Congress.

Kelly Ward Burton: It started as large document dumps, where Guccifer 2.0 was kind of taunting and saying, "I have more."

Kelly Ward Burton was executive director of the DCCC, the Democratic Congressional Campaign Committee, when the Russians hacked the committee's computers.

Kelly Ward Burton: These bullet points at the top are the summary for how we need to win.

They swiped and dumped on the internet material she told us cost millions of dollars to produce: battle plans for congressional races; demographic research on voters; and extensive dossiers on the weaknesses of their own candidates.

Kelly Ward Burton: So when we deliberate internally about anything, you know, that's not intended to be made public. And that's what makes this so important to understand these as stolen documents. It's not different than Watergate. It's not different than when, you know, Republicans came into the DNC and stole documents from the file cabinets. It's the cyber version of that. They came into our office, and they stole our documents. Documents that were never intended to be public. And then they used that in the election.

Kelly Ward Burton

Ward was shocked when Republicans used the stolen internal materials in a negative ad.

Kelly Ward Burton: We reached out to them and asked them. You know, we-- we said, "We have been the victims of a cyberattack by a foreign adversary. Will you make a commitment not to use any of these stolen materials in the-- in the campaign, or in the 2016 election?" And they wouldn't make a commitment to do so.

She says in the months leading up to the elections, Russian tactics evolved. The indiscriminate document dumps became more frequent and strategic.

Kelly Ward Burton: There would be thousands of documents that would show up on one day and then they got smarter, and they started to release specific documents related to our specific races, or documents that were, you know, in our most-targeted states and our most-targeted areas.

The Russian agents stole material about candidates running for Congress in Pennsylvania, New Hampshire, Ohio, Illinois, Nebraska, New Mexico and North Carolina. But one swing state seemed to be the Kremlin's primary target: Florida.

In 2016, Annette Taddeo was running for Congress in the 26th District, which stretches from South Miami to the Florida Keys, one of the most hotly contested races in this battleground state. Taddeo had the full backing of the DCCC. But her campaign was upended two weeks before the primary.

Annette Taddeo: I was on my way to a TV debate, live TV debate, and I get the call about the fact that not only were we hacked, but our information is now public, from our polling to our mail plan. In addition to that, the entire "Path to Victory."

Bill Whitaker: It's your gameplan?

Annette Taddeo: Yes. My opponent, Joe Garcia, showed up at that debate with a printout of all the documents.

Her primary opponent, a fellow Democrat, used the hacked material as a prop to paint her as a conniving politician. The same day, Guccifer 2.0 dropped this mocking post: "the congressional primaries are also becoming a farce." Taddeo lost the primary. Garcia went on to lose the election to the Republican candidate.

Bill Whitaker: You describe South Florida as rough and tumble. But this seems to ratchet it up a notch?

Annette Taddeo: We've seen a lot here. But this was, this was a foreign government. This was so much bigger. You know, I've been told by a lotta people, "You should stop talking about this. It's really not good for you politically to remind people that you lost." But I refuse to stop talking about it. Because, again, if it can happen to me, it can happen to anyone. And it didn't happen to me: It happened to our democracy.

Bill Whitaker: You lost by how much?

Annette Taddeo: About 700 votes.

Marc Caputo: This is a state where elections are decided by a percentage point or so. A coin toss. Add the Russians onto that and you're looking at a real problem.

Marc Caputo has covered Florida politics for 20 years. The senior writer for Politico was one of the reporters who received and wrote about the hacked documents.

Bill Whitaker: Not a lot of people know that the Russians interfered in five congressional races here in Florida. When did you first get wind of it?

Marc Caputo: Well, I'd been paying attention, like the rest of the press corps, that Russia had been hacking and Russia had been trying to interfere in our election system. And then out of the blue I got contacted by this blogger, Hello Florida.

The blogger turned out to be this man, Aaron Nevins, one of the shadier political operators in the Sunshine State. The Republican strategist wouldn't talk to us on camera, but he did talk to special counsel Robert Mueller's investigators. He admits direct-messaging Guccifer 2.0, asking for any Florida-related documents. Seeing a willing participant, the Russians flooded Nevins with hacked materials. "Holy 'F' man," he responded. "I don't think you realize what you gave me… this is probably worth millions of dollars." Guccifer 2.0 responded: "OK, you owe me a million" with a smiley face. Nevins posted the stolen documents on his website, organized in files, and alerted Florida journalists who couldn't resist publishing the Democrats' secrets. At one point Nevins wrote the Russians: "I honestly think you helped sink Annette Taddeo in Florida 26."

Bill Whitaker: You played a role in disseminating this stolen information.

Marc Caputo: I have a role to play as a reporter covering campaigns. And sometimes that information comes to us from a variety of sources. And in this case, it came to us from a source right at the edge of being unusable. But ultimately we decided, "Well, this tells a legitimate story about how these campaigns view their own candidates." And voters have a right to that information.

Robert Anderson: This operation was a huge success.

Former FBI spy-hunter Robert Anderson says Russia's goals today are the same as in the Soviet era: to sow discord in the U.S. and doubt about our democracy around the world.

Robert Anderson: The thing that you need to worry about with Russia and every one of their intelligence services is they will learn from these operations. They'll learn how easy it is to gain access to government and private accounts. They'll learn how quickly the information that they put in front of somebody will be disseminated. They will analyze everything they did right or wrong. And when they attack again, they will not come at you the same way.

Produced by Graham Messick. Associate producer, Jack Weingart. Broadcast associate, Emilio Almonte.