One of the Russian-bought advertisements that Facebook shared with congressional investigators on Monday featured photographs of an armed black woman "dry firing" a rifle — pulling the trigger of the weapon without a bullet in the chamber, according to people familiar with the investigation.

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, speaking on condition of anonymity to discuss the probe. But the precise purpose of the ad remains unclear to investigators, the people said.

The apparent tactic underscores how the Russians used U.S.-based technology platforms to target Americans with highly tailored and sometimes-contradictory messages to exploit divisions in American society over the past two years.

The ad was among more than 3,000 Facebook ads delivered to congressional investigators that the company says were bought by 470 accounts and pages controlled by a Russian troll farm, the Internet Research Agency, based in St. Petersburg.

The full universe of words and images in those ads has not yet been made public, but early glimpses reported in The Washington Post and other news outlets showed that the Russian campaign frequently sought to widen existing fractures in American society, while also helping to boost Republican Donald Trump's presidential campaign.

Among the other Facebook ads shared with lawmakers are those featuring photos of Hillary Clinton behind what appear to be prison bars. This echoed calls by Trump and his supporters during campaign events to "Lock Her Up" — imprison Clinton for using a private email server while she was secretary of state.

The Russian disinformation campaign included ads with harsh language and imagery about illegal immigrants. Others highlighted civil rights groups such as Black Lives Matter and support among Muslim voters for Clinton.

"These ads are racist propaganda, pure and simple," said Malkia Cyril, a Black Lives Matter activist in Oakland, Calif., and executive director for the nonprofit Center for Media Justice. "Whether they appear to be in support or in opposition to black civil rights is irrelevant. Their aim is to subvert democracy for everyone by using anti-black stereotypes — an idea as old as America."

The tools developed by Facebook and other American companies in recent years have given advertisers unprecedented power to identify people susceptible to their messages and to repeatedly deliver targeted advertisements to them over weeks or months as they browse the Internet.

The release to Congress is part of a widening government probe into how Russian operatives used Facebook, Twitter, Google and other technology platforms. 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.

In addition to sharing the ads, Facebook is providing information to lawmakers about which users those ads targeted, the views and clicks the ads received, and the methods of payment used by the Russian operatives, said people familiar with the investigation. The ads were viewed tens of millions of times, people familiar with the investigation said.

"The big picture is that we're stepping up tomorrow to help Congress understand foreign interference on the ad platform and to make improvements to the ad platform to enhance transparency," said Joel Kaplan, Facebook's vice president for U.S. public policy, in an interview Sunday. "We're committed to doing our part to prevent this type of malicious interference."

The company has acknowledged that many of the ads posted by the Russian operatives to sow cultural divisions would not be covered by new safeguards that would apply to ads that mention candidates.

Facebook has said most of the ads bought by the 470 pages and accounts associated with the Internet Research Agency focused on social issues and did not name candidates.

The company also said it would disclose more information about political ads on Facebook, and was adding security measures to crack down on foreign meddling in its ad systems. Facebook provided few details on what those security measures would entail, except to say that it was adding new documentation requirements in which advertisers will have to certify that they are legitimate political organizations before they buy ads. Facebook also said it is planning to hire an additional 1,000 ad reviewers in the coming months. (Facebook is also in the process of hiring thousands of reviewers to examine other types of content, such as hate speech and live video.)

Facebook will now require every group that runs political ads on the social network to publicly post copies of all the ads they have purchased. This change, which Facebook chief executive Mark Zuckerberg announced earlier, is one for which open government advocates have long clamored.

Those advocates have said it is unfair that, unlike broadcasters, Facebook and other online companies are not subject to federal requirements that they disclose the names of groups that are buying political ads on their platforms and the amount they spend, even as social media occupies a growing share of advertising budgets for political campaigns and political advertisers become some of the largest advertisers on Facebook.

Facebook's decision to disclose the actual content of the ads goes further than the requirements for broadcasters.

Political campaigns currently spend about 25 to 35 percent of their ad budgets online, with most going to Google and Facebook, said Zac Moffatt, chief executive of the Republican-leaning digital strategy firm Targeted Victory. Moffatt said that Facebook's decision to publish the content of ads will push other online companies to do the same, and could also help Facebook's profits because giving political advertisers visibility into spending online will encourage them to make bigger purchases in order to match their rivals.

The move by Facebook is a departure from its standard practice, in which Facebook users can only see ads and posts that the company's software algorithm targets directly to them.

Twitter said last week that it has also shut down 201 accounts that were tied to the same Russian operatives who posted thousands of political ads on Facebook. Google is conducting its own internal investigation into Russian meddling.

Dwoskin reported from San Francisco. Matea Gold and Greg Miller contributed to this report.