Facebook CEO Mark Zuckerberg testified to a joint hearing of the Senate Judiciary and Commerce committees Tuesday that Facebook is cooperating with special counsel Robert Mueller's investigation.
"I know we are working with them," Zuckerberg said.
When asked by Sen. Patrick Leahy, D-Vt., about Facebook's involvement with Mueller's investigation, Zuckerberg confirmed the cooperation but refused to divulge additional information.
"I want to be careful here, because that work is confidential. We are in open session and I don't want to reveal anything that is confidential," Zuckerberg said.
The CEO clarified that he had not personally been interviewed by Mueller's team but that others in the company had. When asked if Facebook had been issued a subpoena, Zuckerberg first said it had, before backpedaling.
"I have to clarify that I'm not sure that there are subpoenas, there may be," he said.
The company is facing questions following reports that research firm Cambridge Analytica improperly gained access to the personal data of as many as 87 million Facebook users.
Zuckerberg is likely to face tough questions on user privacy, foreign meddling on the site and abuse of social media tools.
He's scheduled to testify before the House Energy and Commerce Committee Wednesday. His prepared remarks for that hearing were released Monday.
Watch the live stream of Zuckerberg's testimony.