Facebook is facing an unusual degree of scrutiny as Robert Mueller’s team of prosecutors makes the social network a central focus of the Justice Department’s probe into Russian interference in the 2016 election, including how the platform was used to disseminate foreign propaganda and misleading news stories. Earlier this month, Facebook told congressional investigators that it sold about $100,000 worth of ads to a pro-Kremlin Russian troll farm that targeted U.S. voters. But while some lawmakers appeared frustrated by Facebook’s overly general answers to their inquiries, Mueller isn’t asking nicely.

On Friday, CNN reported that the special counsel obtained a search warrant for copies of the Russia-linked ads. According to The Wall Street Journal, the information includes details about the fake accounts that bought the ads and the targeting criteria that was used to reach U.S. voters. They may serve to provide Mueller with a more detailed picture of how the purchased ads helped influence voter sentiment during the 2016 election.

The latest revelation could mark a turning point in Mueller’s investigation. In order to obtain a search warrant, the former F.B.I. director would have had to prove that he has evidence suggesting a crime occurred and that it occurred on Facebook. “He would have to sort of lay out evidence showing that this crime had occurred, not just merely say so, but records that he had obtained, testimony that had been given, or interviews that people gave to the F.B.I.,” former federal prosecutor Renato Mariotti told CBS News on Sunday. “It's a very serious and significant move forward for the Mueller investigation.” Anyone who was part of that effort could be criminally liable, he added. Because Mueller has been looking at relatively specific, narrow crimes, Mariotti said he believes the special counsel’s office is “closing in on charging foreign individuals.” As Chris Smith wrote for Vanity Fair on Friday, some lawmakers believe that investigation could include a closer look at the election data operation run by Jared Kushner and Trump’s digital campaign chief, Brad Parscale, as well as their work with the data-mining firm Cambridge Analytica.

Facebook hasn’t provided the same ads to members of the House and Senate Intelligence Committees, citing its privacy policy in accordance with the federal Stored Communications Act. And some elected officials seem to be losing their patience. Rep. Adam Schiff, the top Democrat on the House Intelligence Committee, said Sunday on ABC’s This Week that “a lot of unanswered questions” about Facebook and the Russian-linked ads remain, and that his committee is planning to ask for more information from Facebook. “We need to know the full extent of their use of social media to influence us from Facebook, from Twitter, from Google, from any social media or search engine,” Schiff said. “They need to be fully forthcoming. And I’m confident they will. I think, frankly, they need to come and testify before Congress because there’s a lot we need to know about this.”

The fallout from the 2016 election continues to be a political problem for Facebook, which is under unprecedented pressure on Capitol Hill from both sides of the aisle. Republicans, who have always been skeptical of Facebook’s political biases, are reportedly investigating whether the social network poses national security threats and are looking at ways to rein it in. And Democrats are working on legislation to force Facebook to disclose more information about the political ads it serves online. Senate Intelligence Committee Chairman Richard Burr, a North Carolina Republican, has said that it’s “probably more of a question of when” than if Facebook officials will have to testify. The committee’s top Democrat, Senator Mark Warner, has argued that “the whole notion of social media and how it is used in political campaigns is the wild wild west.” He added that Facebook’s earlier denials “in the immediate aftermath of our elections” raise questions about what the company knew, and when it knew it.