Washington (CNN) Facebook CEO Mark Zuckerberg confirmed Tuesday that his company is cooperating with special counsel Robert Mueller in its investigation into Russian meddling in the 2016 election.

When asked by Democratic Sen. Patrick Leahy while he testified on Capitol Hill whether Facebook had been served subpoenas for the special counsel, Zuckerberg responded "yes," but later clarified: "I am actually not aware of a subpoena. I'm aware that there may be, but we are working with them."

When asked if his employees had been interviewed, he also responded yes, but added, "I have not."

He continued: "I want to be careful here because 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 is confidential."

His appearance at a joint hearing of the Senate Commerce and Judiciary committees marks the first time that Zuckerberg has testified before Congress. The hearings come nearly a month after news broke that Cambridge Analytica, a data firm with ties to President Donald Trump's campaign, accessed information from as many as 87 million Facebook users without their knowledge.

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