Russia's most inflammatory social media misinformation posts weren't paid advertisements, according to a new report commissioned by the Senate Select Committee on Intelligence.

Accounts run by the Russian-backed Internet Research Agency (IRA) — at the center of Russia's online efforts to interfere in U.S. presidential and congressional elections — saw far more traction with organic social media posts that purported to come from average American citizens, researchers said.

The report sheds light on the scale and scope of Russian social media campaigns, which have long been discussed in terms of ad spend and promoted posts. Facebook and Twitter first disclosed Russian-bought ads last fall, revealing posts paid for in rubles and ratcheting up the number of users who saw the advertisements in the months that followed.

The companies subsequently tweaked their advertising platforms to prevent such abusive targeting.

But, "the most far reaching IRA activity is in organic posting, not advertisements," Oxford researchers said in the report, which was released Monday.

"Facebook now focuses on ad transparency, while disabling the API for public posts ... However, in this report we found that the IRA's political ad activity has not particularly increased over time, while organic post activity has."

IRA accounts leveraged divisive social issues and inflammatory images to garner tens of millions of social media impressions, the report says. The IRA posted memes and images that frequently "expressed tolerance of extremist views," targeted marginalized groups like immigrants, African-Americans and LGBTQ communities. The accounts sought to pit groups against each other and stir online debate.

Facebook provided researchers with roughly 3,300 ads, and more than 180,000 organic posts produced by IRA pages across Facebook and Instagram. Twitter provided the researchers with more than 8 million tweets across 3,800 accounts.

The report, one of two that were released Monday, is the most comprehensive study yet of the misinformation efforts. Researchers at Oxford University's Internet Institute reviewed data provided by Facebook, Twitter and Google to determine the targets and tactics of the Russian-backed accounts.