Facebook CEO Mark Zuckerberg Mark Elliot Zuckerberg2.5 million US users register to vote using Facebook, Instagram, Messenger Hillicon Valley: Trump's ban on TikTok, WeChat in spotlight | NASA targeted by foreign hackers | Instagram accused of spying in lawsuit The Hill's Morning Report - Sponsored by The Air Line Pilots Association - Trump contradicts CDC director on vaccine, masks MORE said Tuesday that special counsel Robert Mueller Robert (Bob) MuellerCNN's Toobin warns McCabe is in 'perilous condition' with emboldened Trump CNN anchor rips Trump over Stone while evoking Clinton-Lynch tarmac meeting The Hill's 12:30 Report: New Hampshire fallout MORE has interviewed Facebook employees as part of his investigation into Russian election interference.

The social media executive also said he “believes” the company may have been served with a subpoena from the special counsel’s office.

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Zuckerberg made the disclosure in response to Sen. Patrick Leahy Patrick Joseph LeahyBattle over timing complicates Democratic shutdown strategy Hillicon Valley: Russia 'amplifying' concerns around mail-in voting to undermine election | Facebook and Twitter take steps to limit Trump remarks on voting | Facebook to block political ads ahead of election Top Democrats press Trump to sanction Russian individuals over 2020 election interference efforts MORE (D-Vt.), who asked whether Facebook employees had been interviewed as part of the investigation.

"Yes," Zuckerberg said, clarifying he had not personally been interviewed.

It was previously known that Facebook handed over information to Mueller on political advertisements purchased by Russia-linked accounts, but there have been few public details of the extent of the social media’s cooperation in the investigation.

The Facebook CEO on Tuesday declined to disclose many details, citing the sensitive nature of the ongoing investigation.

“I want to be careful here because our work with the special counsel is confidential,” Zuckerberg told lawmakers. "I actually am not aware of a subpoena; I believe that there may be."

“I know that we are working with them," he added.

Facebook revealed last September that the company unknowingly sold $100,000 in political ads to accounts associated with the Internet Research Agency, a group based in St. Petersburg that aimed to use social media to distribute divisive content to U.S. audiences ahead of the presidential election.

Mueller has indicted 13 Russians and three Russian entities with links to the Internet Research Agency operation on conspiracy and fraud-related charges. Mueller is investigating Russian interference in the 2016 election, including whether there was collusion between President Trump Donald John TrumpBubba Wallace to be driver of Michael Jordan, Denny Hamlin NASCAR team Graham: GOP will confirm Trump's Supreme Court nominee before the election Southwest Airlines, unions call for six-month extension of government aid MORE's campaign and Moscow.

Zuckerberg testified at a joint hearing of the Senate Commerce and Judiciary committees Tuesday afternoon amid scrutiny over the company's handling of the Cambridge Analytica scandal.