The batch of more than 3,000 Russian-bought ads that Facebook is preparing to turn over to Congress shows a deep understanding of social divides in American society, with some ads promoting African American rights groups, including Black Lives Matter, and others suggesting that these same groups pose a rising political threat, say people familiar with the covert influence campaign.

The Russian campaign — taking advantage of Facebook's ability to send contrary messages to different groups of users based on their political and demographic characteristics — also sought to sow discord among religious groups. Other ads highlighted support for Democrat Hillary Clinton among Muslim women.

These targeted messages, along with others that have surfaced in recent days, highlight the sophistication of an influence campaign slickly crafted to mimic and infiltrate U.S. political discourse while also seeking to heighten tensions between groups already wary of one another.

The nature and detail of these ads have troubled investigators at Facebook, on Capitol Hill and at the Justice Department, say people familiar with the advertisements, who spoke on the condition of anonymity to share matters still under investigation.

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The House and Senate intelligence committees plan to begin reviewing the Facebook ads in coming weeks as they attempt to untangle the operation and other matters related to Russia's bid to help elect Donald Trump president in 2016.

"Their aim was to sow chaos," said Sen. Mark R. Warner (D-Va.), vice chairman of the Senate Intelligence Committee. "In many ­cases, it was more about voter suppression rather than increasing turnout."

The top Democrat on the House Intelligence Committee, Rep. Adam B. Schiff of California, said he hoped the public would be able to review the ad campaign.

"I think the American people should see a representative sample of these ads to see how cynical the Russians were using these ads to sow division within our society," he said. Schiff had not yet seen the ads but was briefed on them, he said, including the ones mentioning "things like Black Lives Matter."

The ads that Facebook found raise troubling questions for a social networking and advertising platform that reaches 2 billion people each month, and they offer a rare window into how Russian operatives carried out their information operations during an especially tumultuous period in U.S. politics.

[Mark Zuckerberg denies that fake news on Facebook influenced the election]

Investigators at Facebook discovered the Russian ads in recent weeks, the company has said, after months of trying in vain to trace disinformation efforts to Russia. The company said it has identified at least $100,000 in ads purchased through 470 phony Facebook pages and accounts. Facebook said this spending represented a tiny fraction of the political advertising on the platform during the 2016 campaign.

The divisive themes seized on by Russian operatives were similar to those that Trump and his supporters pushed on social ­media and on right-wing websites during the campaign. U.S. investigators are now trying to figure out whether Russian operators and members of Trump's team coordinated in any way. Critics say Trump, as president, has further inflamed racial and religious divisions, citing his controversial statements after violent clashes in Charlottesville and limits imposed on Muslim immigration.

The previously undisclosed ads suggest that the operatives worked off evolving lists of racial, religious, political and economic themes. They used these to create pages, write posts and craft ads that would appear in users' news feeds — with the apparent goal of appealing to one audience and alienating another. In some cases, the pages even advertised events.

"The idea of using Facebook to incite anti-black hatred and anti-Muslim prejudice and fear while provoking extremism is an old tactic. It's not unique to the United States, and it's a global phenomenon," said Malkia Cyril, a Black Lives Matter activist in Oakland, Calif., and executive director for the Center for Media Justice. Social media companies "have a mandate to stand up and take deep responsibility for how their platforms are being abused."

Facebook declined to comment on the contents of the ads being turned over to congressional investigators and pointed to a Sept. 6 statement by Alex Stamos, the company's chief security officer, who noted that the vast majority of the ads run by the 470 pages and accounts did not specifically reference the U.S. presidential election, voting or any particular candidate.

"Rather, the ads and accounts appeared to focus on amplifying divisive social and political messages across the ideological spectrum — touching on topics from LGBT matters to race issues to immigration to gun rights," ­Stamos said at the time.

A Cold War tactic

Moscow's interest in U.S. race relations dates back decades.

In Soviet times, operatives didn't have the option of using the Internet, so they spread their messages by taking out ads in newspapers, posting fliers and organizing meetings.

Much like the online ads discovered by Facebook, messages spread by Soviet-era operatives were meant to look as though they were written by bona fide political activists in the United States, disguising the involvement of an adversarial foreign power.

Russian information operations didn't end with the collapse of the Soviet Union.

After a lull in tensions, Russia's spy agencies became more assertive under the leadership of President Vladimir Putin. In recent years, those services have up­dated their propaganda protocols to take advantage of new technologies and the proliferation of ­social media platforms.

"Is it a goal of the Kremlin to encourage discord in American society? The answer to that is yes," said Michael A. McFaul, a former U.S. ambassador to Russia who is now a director of the Freeman Spogli Institute for International Studies at Stanford University. "More generally, Putin has an idea that our society is imperfect, that our democracy is not better than his, so to see us in conflict on big social issues is in the Kremlin's interests."

Clinton Watts, part of a research team that was among the first to warn publicly of the Russian propaganda campaign during the 2016 election, said that identifying and exploiting existing social and cultural divisions are common Russian disinformation tactics dating back to the Cold War.

"We have seen them operating on both sides" of an issue, said Watts, a fellow with the Foreign Policy Research Institute and a former FBI agent.

Microtargeting users

When Mark Zuckerberg founded Facebook in his college dorm room in 2004, no one could have anticipated that the company would become an advertising juggernaut worth almost half a trillion dollars — the largest online advertising company in the world after Google. Roughly a third of the world's population now logs in monthly.

As Facebook's user base rapidly expanded, the company wrote the playbook for digital targeting in the smartphone era — and for the type of microtargeting that has become critical to modern political campaigns.

The social network invested heavily in building highly sophisticated automated advertising tools that could target specific groups of people who had expressed their preferences and interests on Facebook, from newlyweds who studied at Dartmouth College to hockey enthusiasts living in a particular Zip code in Michigan.

The migration from traditional personal computers to smartphones and tablets helped Facebook gain a major edge: The company pioneered techniques to help advertisers reach the same user on their desktop and mobile devices, helping Facebook grow sevenfold in value since it went public in 2012. Today, advertisers who want to target Facebook ­users by demographics or interests have tens of thousands of categories to choose from, and they are able to flood users with ads wherever they go on the Internet.

Ads on Facebook have directly appeared in people's news feeds since 2012. If a user "likes" a page, administrators of that page can pay for ads and post content that will then appear in that person's news feed.

Since the 2012 presidential election, Facebook has become an essential tool for political campaigns that wish to target potential voters. During the height of election season, political campaigns are among the largest advertisers on Facebook. Facebook has built a large sales staff of account executives, some of whom have backgrounds in politics, who are trained to assist campaigns in spreading their messages, increasing engagement and getting immediate feedback on how they are performing.

The Trump campaign used these tools to great effect, while Clinton's campaign preferred to rely on its own social media experts, according to people familiar with the campaigns.

Aiming at swing voters

Since taking office, Putin has on occasion sought to spotlight racial tensions in the United States as a means of shaping perceptions of American society.

Putin injected himself in 2014 into the race debate after protests broke out in Ferguson, Mo., over the fatal shooting of Michael Brown, an African American, by a white police officer.

"Do you believe that everything is perfect now from the point of view of democracy in the United States?" Putin told CBS's "60 Minutes" program. "If everything was perfect, there wouldn't be the problem of Ferguson. There would be no abuse by the police. But our task is to see all these problems and respond properly."

In addition to the ads described to The Post, Russian operatives used Facebook to promote anti-immigrant and anti-Muslim messages. And Facebook has said that one-quarter of the ads bought by the Russian operatives identified so far targeted a particular geographic area.

[Did Facebook ads traced to a Russian company violate U.S. election law?]

While Facebook has played down the impact of the Russian ads on the election, Dennis Yu, chief technology officer for BlitzMetrics, a digital marketing company that focuses on Facebook ads, said that $100,000 worth of Facebook ads could have been viewed hundreds of millions of times.

According to Yu, "$100,000 worth of very concentrated posts is very, very powerful. When you have a really hot post, you often get this viral multiplier. So when you buy this one ad impression, you can get an extra 20- to 40-times multiplier because those people comment and share it."

Momentum is building in Congress and elsewhere in the federal government for a law requiring Facebook and other Web companies to reveal publicly who bought political ads and the amount that was spent on their platforms. Newspapers, television stations and other traditional carriers of campaign messages already disclose such information.

Watts, the Foreign Policy Research Institute fellow, said he has not seen the Facebook ads promised to Congress, but he and his team saw similar tactics playing out on Twitter and other platforms during the campaign.

Watts said such efforts were most likely to have been effective in Midwestern swing states such as Wisconsin and Michigan, where Democratic primary rival Sen. Bernie Sanders had beaten Clinton. Watts said the disinformation pushed by the Russians included messages designed to reinforce the idea that Sanders had been mistreated by the Democratic Party and that his supporters shouldn't bother to vote during the general election in November.

"They were designed around hitting these fracture points, so they could see how they resonate and assess their effectiveness," Watts said. "I call it reconnaissance by social media."

Dwoskin reported from San Francisco. Tom Hamburger contributed to this report.