Facebook CEO Mark Zuckerberg confirmed on Tuesday that the social media giant is working with special counsel Robert Mueller’s office on his investigation into Russian meddling in the 2016 election.

Speaking before a joint hearing of the Senate Judiciary and Commerce committees, Zuckerberg declined to make clear the extent of Facebook’s cooperation with Mueller.

"I know we are working with them," Zuckerberg said, vaguely confirming statements made on behalf of the social network in September.

Democratic Sen. Patrick Leahy of Vermont asked Zuckerberg a string of questions aimed at teasing out the particulars.

“I assume Facebook’s been served subpoenas from the special councilman's office,” Leahy said. “Is that correct?”

“Yes,” Zuckerberg said.

But when Leahy asked Zuckerberg to confirm that he had been subpoenaed, the Facebook CEO backtracked.

“Actually, let me clarify that: I actually am not aware of a subpoena,” Zuckerberg said. “I believe that there may be, but I know we’re working with them.”

Leahy asked if anyone at Facebook has been interviewed by the special counsel's office, which Zuckerberg confirmed. Leahy then asked if Zuckerberg himself had been interviewed, and the CEO said he hadn’t — only other people at Facebook had been.

“I believe so, and I want to be careful here because that… our work with the special counsel is confidential and I want to make sure that in an open session I’m not revealing something that’s confidential,” Zuckerberg said.

Facebook confirmed months ago that it provided Mueller’s team with detailed information related to Russian ad buys, and previous reports indicate Mueller’s office has interviewed at least one member of Facebook’s team, who was associated with President Trump's 2016 presidential campaign. But this is the first time the Facebook CEO has personally and publicly verified their involvement.

The social media giant is under fire in the wake of reports that over 80 million people, mostly located in the U.S., had their data “improperly shared” with the data mining firm Cambridge Analytica. The Robert Mercer-backed company collected these profiles without users' consent via third-party apps, ultimately using that data to target content in ways designed to help Donald Trump’s campaign.

Tuesday was tough for Zuckerberg, but the questions on user privacy, foreign meddling on the site, and abuse of social media tools, aren’t close to finished. He's scheduled to testify before the House Energy and Commerce Committee Wednesday.