Black lawmakers in Congress are increasing pressure on Facebook and Twitter to prevent Russian malefactors — or anyone else — from spreading racist messages on social media in a bid to fuel political unrest in the United States.

During the 2016 presidential election, Kremlin-aligned forces purchased thousands of ads on Facebook, many of which sought to stir trouble by riling supporters and opponents alike on both sides of contentious issues — including causes like immigration and Black Lives Matter, sources have said.

Given the tech industry’s well-documented troubles addressing issues of diversity, three U.S. lawmakers — Reps. Robin Kelly, Bonnie Watson Coleman, and Emanuel Cleaver — are now pushing the two tech giants anew to police their platforms more aggressively.

In a letter sent Friday, obtained by Recode, they urge Facebook and Twitter to appoint people of color to their boards of directors, while commissioning new audits of ads at each company to ensure they aren’t spreading “fake news.” And the three lawmakers — all members of the Congressional Black Caucus, or CBC — further press the tech giants to take down ads that are “aimed at inciting racial discontent” or “voter suppression.”

With it, they ask Facebook and Twitter to commit to sharing copies of the ads purchased by Russian sources during the presidential election with all members of Congress by December 1, 2017. So far, Facebook has found about 3,000 ads bought by Kremlin sources, while Twitter has found about 200 accounts tied to Russia. But the two companies have only shared that information with the three congressional committees investigating Kremlin meddling in the election.

“As members of the House of Representatives, we owe it to the communities we represent to ensure that social media platforms are not manipulated to incite violence, sow discord, or undermine our democratic institutions,” Kelly, Watson Coleman, and Cleaver begin. “Members of Color, in particular, are additionally impacted by this issue, as the communities we represent are disproportionately strong consumers of social media, and additionally vulnerable to these attacks and misinformation.”

Spokespeople for Facebook and Twitter did not immediately respond to requests seeking comment.

To be sure, Facebook and Twitter have rules in place that prohibit discrimination of any sort — from the ads that groups purchase to the posts that their uses share.

But abusive content often seems to slip through the cracks, and the companies’ ad systems have been easily gamed in the past — such as when ProPublica found itself able to purchase and target advertising on Facebook aimed at those interested in “Jew haters.”

Following those incidents, the companies have further tightened their systems. And as reports emerged about the extent of Russia’s attempts to spread disinformation in 2016 by preying on users’ racial, religious or other characteristics, the tech giants promised additional checks still to come. This week, for example, Facebook informed advertisers that it would begin subjecting more of their content to manual review.

Some of the commitments have drawn some early, tentative praise from the three black lawmakers — Kelly, Watson Coleman, and Cleaver — who wrote Facebook and Twitter on Friday. But they still stress in their letter that there’s much more work to do, particularly in light of the fact that “the next foreign-sponsored effort to disrupt U.S. elections … will not resemble the last,” they wrote.

Behind the scenes, meanwhile, CBC members contacted Facebook and Twitter in recent weeks to raise their concerns about the Russian-bought divisive ads. The meeting came after some of the same lawmakers targeted Twitter, specifically, in a letter urging it to address the racist content posted online — by users, bots and Russian trolls alike.

Nor is it the first time that the Congressional Black Caucus has pressed the tech industry on issues related to diversity. For years, its leaders have urged companies like Apple, Facebook, Google and Twitter to improve their hiring practices, similarly threatening regulation and other scrutiny if they did not.

Facebook and Twitter could face some of these questions in the open next month, when they’re set to appear before the House and Senate Intelligence Committees to testify about Russian meddling in the 2016 election.