ST. PETERSBURG, Russia  — The four-story building at 55 Savushkina St. that housed the infamous Russian internet "troll factory" that meddled in America's 2016 election now appears empty. I saw a huge "For Rent" sign in one of its large windows facing the street.

The main trolling operation has moved to an impersonal seven-story glass office building in the distant Lakhta business district. I couldn't enter the building due to tight security. (Journalists seen taking pictures have been grabbed and harassed.) The city's leading business daily Delovoy Peterburg reported late last year that the operation's workspace has tripled.

Yet, little known to Americans, this fake news factory, whose organizers and some staff were indicted by special counsel Robert Mueller for interfering with  the U.S. political system, was not initially intended to wage information warfare on America.

Instead, its original purpose, starting in 2014, seems to have been to shape the thinking of ordinary Russians by viciously attacking domestic opposition activists and fueling public hostility toward Ukraine – and the United States.

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The domestic propaganda barrage continues out of the troll factory's new digs, which include a massive online media conglomerate that intertwines news with Kremlin propaganda. "A tricky mix of normal reporting and hate speech," one Russian journalist calls it. RBC magazine wrote in March 2017 that this "media factory" now reaches an audience that exceeds 50 million Russians a month.

 In May, this "media factory" will launch a new "information agency" called "USA Really. Wake Up Americans." It has already put out an announcement that  it "will  focus on promoting information and problems that are hushed up by major American publications controlled by the U.S. political elite." The agency is inviting "English-speaking journalists and authors to work on the project " and send their CVs.

Now that its secret internet project has been exposed,  the  troll factory will apparently focus more on open information warfare. Kremlin propaganda mills masquerading as  news agencies will try to insert anti-American propaganda into U.S. – and Russian – media.

Given the rising tensions between Moscow and Washington, I think the domestic trolling operation holds more potential threat to America – than any (over-hyped) efforts to subvert U.S. democracy. It promotes Kremlin efforts to convince Russians that the United States wants to destroy their country and start WWIII (enhancing similar messages on state-controlled TV).

Since at least 2014, this trolling operation has been posting grotesque fabrications to foster Russian support for the Kremlin's invasion of eastern Ukraine — and to whip up hostile feelings toward America. The information operation is funded by a Putin-friendly oligarch, Yevgeny Prigozhin, who just also happens to control a mercenary outfit known as Wagner that has contract troops in Ukraine and Syria.

To find out how the trolling worked, I talked to brave Russian journalists and undercover activists in Moscow and St. Petersburg who have been reporting on the fake news factory since long before Americans ever heard of Russian trolls.

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Vitaly Bespalov, a 27-year-old freelance journalist with long blond hair and a tattoo of Russian opposition politician Ksenia Sobchak on one arm, came to St.Petersburg in 2014 to try to find work.  He answered an ad for a "content manager" at 55 Savushkina St. and was offered a salary about a third higher than a normal starting job in Russia. It was not long after Russia invaded eastern Ukraine and he was told he would be working for a department called Ukraine II.

Bespalov had to pull  20 news stories about Ukraine daily from the internet and rewrite them to fit the Russian propaganda version. Ukrainians resisting the Russian invasion were "terrorists" and Russia-backed separatists were "volunteers."

When a Ukrainian rocket hit an empty school where "volunteers" were hidden, the news was that Ukrainians wanted to kill children. Americans wanted to use the uprising as an excuse to start World War III.

(According to the BBC's Russian service, the troll factory was linked to a video operation in Ukraine that produced gems like a supposed U.S. soldier shooting bullets into a Quran, clips that might enrage Russian Muslims in the Caucasus.)

After a few days, Bespalov realized what was going on and decided to "stay and get more information and make it public."  He was later assigned to distribute links to fake news stories to social-media groups in different Russian cities. These stories were filled with clichés, such as "[Secretary of State] Clinton wants war with Russia" and nasty claims about President Barack Obama.

"I used to think this work was absolutely stupid work with no results," Bespalov told me. But later he heard ordinary people using these clichés in their conversation "not just about Ukraine but about the USA."

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The trolling news operation backs Kremlin efforts to convince Russians that the United States wants to destroy their country. Similar messages are delivered by state-controlled TV.

Unfortunately, I was unable to learn what the American department of the troll factory is up to these days. Russian journalists say that, since 2016 it is much harder to find sources that have worked inside the troll factory. "They are scared and refuse to talk," says Bespalov.

Only a handful of Russian news outlets still cover this story, which can put journalists at severe risk. After Vitaly Bespalov was interviewed by NBC late last year, the main state TV channel attacked him viciously, and his picture appeared on social media with false claims that he was a drug addict.

In the United States, on the other hand, we have plenty of information, overhyped, about Russian trolling. (Russian journalists I met uniformly expressed astonishment at any claims that the troll factory changed the outcome of the 2016 election.)

We know what happened in 2016, and should be fully able to take adequate precautions about any similar efforts. Forewarned about Russian trolling should be forearmed.