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Facebook has talked to special prosecutor Robert Mueller, CEO Mark Zuckerberg said Tuesday during a hearing on Capitol Hill.

During questioning by Sen. Patrick Leahy, a Democrat from Vermont, Zuckerberg said that someone at his company has spoken with Mueller, though he himself hasn't. He declined to give more information, saying he wasn't sure what he could share in an open session. Zuckerberg also wasn't sure about whether Facebook has received a subpoena.

"Our work with the special counsel is confidential," Zuckerberg said.

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Zuckerberg is being questioned by Congress this week about user privacy and how Facebook user data has been misused by third parties like Cambridge Analytica.

Mueller, meanwhile, has been investigating whether collusion with Russia occurred during the 2016 presidential election. His investigation, overseen by the US Justice Department, has targeted people within President Donald J. Trump's administration, as well as Russian social media trolls.

Zuckerberg's social network has been in the hot seat since it was revealed that Russian trolls abused Facebook -- as well as rival platforms Twitter and Google-owned YouTube -- to meddle in the election and sow discord among Americans. On Facebook, the Russians did that by using a combination of both paid ads and organic posts.

But it's the Cambridge Analytica affair that's at the heart of the biggest scandal in Facebook's 14-year history. Personal info from about 300,000 users was originally collected in 2013 for a personality quiz app called This is Your Digital Life, designed by Aleksandr Kogan, a Cambridge University researcher. Because of how Facebook worked at the time, Kogan was able to access data from friends of the quiz takers -- up to 87 million of them -- and share the information with Cambridge Analytica. The data analytics firm then may have used the data to help the Trump campaign during the 2016 presidential election.

Outcry over the misuse of data has caused Facebook to make changes to its site, including launching a tool to notify users if they were affected by Cambridge Analytica's data access. Facebook also announced new privacy settings and a clearer privacy policy and said it's auditing the thousands of apps on its site to make sure it knows how data is being collected. Facebook is facing several lawsuits over the data misuse.

Cambridge Analytica: Everything you need to know about Facebook's data mining scandal.

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