Facebook CEO Mark Zuckerberg is back on Capitol Hill.

A house hearing examining the company's privacy policies and the role Facebook played as Russians intervened in the 2016 election has begun. Zuckerberg testified for around five hours in a Senate hearing on Tuesday.

In that hearing, the 33-year-old apologized several times for Facebook failures and disclosed that his company was "working with" special counsel Robert Mueller in the federal probe of Russian election interference. He also said Facebook was working hard to change its own operations after the harvesting of users' private data by a data-mining company affiliated with Donald Trump's campaign.

Earlier this year Mueller charged 13 Russian individuals and three Russian companies in a plot to interfere in the 2016 presidential election through a social media propaganda effort that included online ad purchases using U.S. aliases and politicking on U.S. soil. A number of the Russian ads were on Facebook.

Much of the effort was aimed at denigrating Democrat Hillary Clinton and thereby helping Republican Trump, or simply encouraging divisiveness and undercutting faith in the U.S. system.

Zuckerberg said Facebook had been led to believe Cambridge Analytica had deleted the user data it had harvested and that had been "clearly a mistake." He said Facebook had considered the data collection "a closed case" and had not alerted the Federal Trade Commission. He assured senators the company would handle the situation differently today.

Separately, the company began alerting some of its users that their data was gathered by Cambridge Analytica. A notification that appeared on Facebook for some users Tuesday told them that "one of your friends" used Facebook to log into a now-banned personality quiz app called "This Is Your Digital Life." The notice says the app misused the information, including public profiles, page likes, birthdays and current cities, by sharing it with Cambridge Analytica.

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