On Tuesday, Facebook CEO Mark Zuckerberg revealed that Facebook is working with special counsel Robert Mueller in his investigation into alleged Russian collusion with Trump campaign officials.

Zuckerberg told Sen. Patrick Leahy (D-VT) that officials at Facebook had been interviewed by investigators from Mueller’s team. Zuckerberg said that while he had not been interviewed by the special counsel he did believe that other officials at Facebook had been.

“Our work with the special counsel is confidential and I want to make sure in an open session that I am not revealing something that is confidential,” Zuckerberg said.

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