Russian operatives set up an array of misleading Web sites and social media pages to identify American voters susceptible to propaganda, then used a powerful Facebook tool to repeatedly send them messages designed to influence their political behavior, say people familiar with the investigation into foreign meddling in the U.S. election.

The tactic resembles what American businesses and political campaigns have been doing in recent years to deliver messages to potentially interested people online. The Russians exploited this system by creating English-language sites and Facebook pages that closely mimicked those created by U.S. political activists.

The Web sites and Facebook pages displayed ads or other messages focused on such hot-button issues as illegal immigration, African American political activism and the rising prominence of Muslims in the United States. The Russian operatives then used a Facebook “retargeting” tool, called Custom Audiences, to send specific ads and messages to voters who had visited those sites, say people familiar with the investigation who spoke on the condition of anonymity to share details from an ongoing investigation.

People caught up in this web of tracking and disinformation would have had no indication that they had been singled out or that the ads came from Russians.

One such ad featured photographs of an armed black woman “dry firing” a rifle — pulling the trigger of the weapon without a bullet in the chamber — the people familiar with the investigation said.

A sign at Facebook headquarters in Menlo Park, Calif. (Ben Margot/AP)

Investigators believe the advertisement may have been designed to encourage African American militancy and, at the same time, to stoke fears within white communities, the people said. But the precise purpose of the ad remains unclear to investigators, the people said.

Another showed an image of Democrat Hillary Clinton behind what appeared to be prison bars.

A Facebook spokesman declined to comment on Russia’s exploitation of the Custom Audiences system. Facebook officials have previously said that they were caught off guard by the Russian propaganda campaign because the accounts, pages and ads appeared to be legitimate.

In addition to Custom Audiences, Russian operatives used other Facebook tools to target groups by demographics, geography, gender and interests, according to the people familiar with the investigation. The Custom Audiences tool differs because it allows advertisers to feed into Facebook’s systems a specific list of users they want to target.

The conclusions of investigators fit those of several independent researchers, who say that the Russian disinformation campaign exploited the core advertising and tracking technologies that Silicon Valley has honed over a decade to serve corporate America — and that are widely available, with few if any restrictions, to political actors in the United States and abroad.

“These are the same methods and sophisticated tools that the pharmaceutical companies were using, that big oil companies were using,” said Philip N. Howard of Oxford University’s Computational Propaganda Project. “This was regular ad technology that regular advertisers use.”

The revelation about the use of Facebook’s Custom Audiences tool, which has not been previously reported, adds to an emerging picture of a Russian effort to shape the U.S. election and sow division using tools built by American technology companies.

And it makes clear that Russians used Facebook to direct their influence campaigns to voters whom they had already tracked and to find new ones wherever they browsed the Internet — even if they used multiple devices such as a smartphone for work or a tablet at home.

Targeted people might also have directed that same disinformation — whether intentionally or not — to people linked to them on social networks, such as their friends on Facebook.

“This means that any American who knowingly or unknowingly clicked on a Russian news site may have been targeted through Facebook’s advertising systems to become an agent of influence — a potentially sympathetic American who could spread Russian propaganda with other Americans,” said Clinton Watts, a fellow at the Foreign Policy Research Institute. “Every successful click gives them more data that they can use to retarget. It feeds on itself and it speeds up the influence dramatically.”

Jonathan Albright, research director of the Tow Center for Digital Journalism at Columbia University, who has studied the links among fake news, Russian propaganda sites and their relationship to Facebook and other social media platforms, said that hundreds of Russian sites were loaded up with ad tracking software, known as cookies, that would allow them to follow any visitor across the Web and onto Facebook.

The Custom Audiences tool enabled Russian advertisers to feed information from those cookies, which are long strings of numbers that advertisers collect, into Facebook’s systems, which could match them with the accounts of particular Facebook users.

The Facebook users were then shown ads featuring divisive topics that the Russians wanted to promote in their Facebook news feeds, which displayed the ads alongside messages from friends and family members.

As targeted users clicked on the Facebook ads, the system would eventually take them to Web pages outside Facebook, where they would be tracked with more-aggressive forms of tracking software, Albright said.

“A lot of this content is simply for tracking,” Albright said. “You need to get people out of the social networks, off the platforms, because that’s the place where you can attach the advanced ad technology.”

Facebook delivered more than 3,000 ads to congressional investigators on Monday. It is also sharing information on which users those ads were designed to target, how many users viewed or clicked on those ads, and the payment methods used by the Russians.

The company said Monday that modeling shows that these ads were seen by roughly 10 million users. An estimated 44 percent were seen before the Nov. 8 election, and the rest were seen afterward.

Twitter disclosed on Thursday that it had shut down an additional 201 accounts associated with the Internet Research Agency, and it said that three accounts connected with the Kremlin-linked news site RT had spent $274,000 on its platform in 2016. Google said last month that the company had not found evidence of Russian meddling and is conducting an internal investigation into the issue.

The revelations come at a moment when investigators are widening their probe into how Russian operatives used Facebook, Twitter, Google and other technology platforms to widen fissures in the United States and spread disinformation during election season. Those companies have come under increasing pressure from Capitol Hill to investigate Russian meddling and are facing the possibility of new regulations that could affect their massive advertising businesses.

“There’s been some thought that the Internet was a goose laying golden eggs, but now there’s a sense that all the eggs are not golden,” said Jonathan Zittrain, faculty director of Harvard’s Berkman Klein Center for Internet and Society.

Political experts have pointed out that during close elections such as the 2016 presidential contest, a small number of votes by people living in certain states can have a disproportionate influence on the outcome. Without a full accounting of where ads were targeted on the Web and who was targeted — as well as how many times those ads were shared within social networks — it is difficult to assess the impact of the Russian influence campaign.

Facebook introduced Custom Audiences in mid-2012, in the middle of the presidential election contest between Mitt Romney and Barack Obama. Advertisers hailed the tool as a major innovation because it enabled them to know the interests of individuals. People who signal interest in a subject by clicking on a link are also considered more impressionable when they are repeatedly targeted by ads.

Custom Audiences also allowed a business to know when consumers had viewed a particular pair of shoes on a website so that they could be repeatedly shown an ad for those shoes on Facebook and elsewhere online. As consumers spent more time on social media, the tool became a driver of Facebook’s ad business — and of the company’s sevenfold increase in value since its initial public offering in 2012.

Timberg and Entous reported from Washington