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.