On Thursday, Facebook C.E.O. Mark Zuckerberg appeared in a rare video livestream to announce that his company would turn over 3,000 ads to Congress that it believes were purchased by a Russian propaganda outlet with the intention of targeting U.S. voters during the 2016 election. It was the latest attempt by Facebook to defuse a P.R. crisis as the Justice Department continues to zero in on the role social media may have played in Russia’s election interference, and also represented a reversal for Facebook, which had resisted broadly sharing information about these ads. Numerous intelligence experts have described Russia’s online campaign as part of a sophisticated attack on the U.S. electoral system that included hacking the Democratic National Committee, the Clinton campaign, and cyber incursions into voter databases and software systems in 39 states, as well as creating thousands of fake online profiles to sow misinformation and aggravate partisan divisions on social media. Recent evidence suggests that Russia may also have taken its efforts offline, using Facebook to orchestrate pro-Trump rallies in more than a dozen cities.

Still, at least one prominent U.S. politician continues to write off both Russia’s election interference and Facebook’s role. “The Russia hoax continues, now it’s ads on Facebook,” President Donald Trump tweeted on Friday morning, dismissing reports of pro-Kremlin groups purchasing Facebook ads. “What about the totally biased and dishonest Media coverage in favor of Crooked Hillary?” He continued: “The greatest influence over our election was the Fake News Media ‘screaming’ for Crooked Hillary Clinton. Next, she was a bad candidate!”

While Trump has repeatedly cast doubt on the assessment of the U.S. intelligence community that Russia worked to tilt the 2016 presidential race in his favor, the Department of Justice is treating the allegation as a potential criminal conspiracy. Special counsel Robert Mueller is said to have a “red-hot” focus on the role social media played in the Russian campaign—and on the president himself. On Wednesday, The Washington Post and The New York Times both reported that Mueller has requested numerous documents related to actions Trump has taken since assuming office, including internal communications related to his firings of Michael Flynn and James Comey; his conversation with Russian Foreign Minister Sergey Lavrov in the Oval Office; as well as information about Paul Manafort, Jared Kushner, and Donald Trump Jr.’s meeting with a Russian lawyer in Trump Tower last summer.

Facebook’s piece of the Russia puzzle has become a growing headache for Zuckerberg, who initially dismissed reports suggesting that the social network had been weaponized in any capacity. This week, he announced that Facebook users will now be able to visit advertisers’ pages to see what they’re advertising, and who they’re targeting. Facebook will also add disclosures to political ads specifying who paid for them. The company will add 250 employees to strengthen election-integrity initiatives, expanding its partnerships with election commissions worldwide and sharing information about existing threats to elections.

The Kremlin has denied any connection to the Russian-bought Facebook ads, which were purchased over the past two years and found to be linked to fake Russian-based accounts. “We don’t know who places ads on Facebook and how,” Russian President Vladimir Putin’s spokesman Dmitry Peskov told reporters. “We have never done it and the Russian side has never had anything to do with it.”