WASHINGTON - Facebook CEO Mark Zuckerberg disclosed Tuesday that his company is working with special counsel Robert Mueller in the federal probe of Russian interference in the 2016 presidential campaign - and working hard to change its own operations after the harvesting of users' private information by a data-mining company affiliated with the campaign of US President Donald Trump.

The founder of the social media giant publicly apologized for his company's errors in failing to better protect the personal information of its millions of users, a controversy that has brought a flood of bad publicity and sent the company's stock value plunging. Facebook shares, did, however increased 4.5 percent on Tuesday, their biggest gain in two years.

Zuckerberg told the Senate Judiciary and Commerce committees that he has not been personally interviewed by Mueller's team, but "I know we're working with them." He offered no details, citing a concern about confidentiality rules of the investigation.

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 US aliases and politicking on US soil. A number of the Russian ads were on Facebook.

During Tuesday's hearing, which at times was contentious, Zuckerberg said it had been "clearly a mistake" to believe the data-mining company Cambridge Analytica had deleted user data that it had harvested in an attempt to sway elections.

He said Facebook had considered the data collection "a closed case" because it thought the information had been discarded. Facebook also didn't alert the Federal Trade Commission, Zuckerberg said, and he assured senators the company would handle the situation differently today.

He began a two-day congressional inquisition with a public apology for the way Facebook handled the data-mining of its users' data. He took responsibility for failing to prevent Cambridge Analytica, which was affiliated with Trump's presidential campaign, from gathering personal information from 87 million users.

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 such as public profiles, likes, birthdays and current cities, by sharing it with Cambridge Analytica.

Zuckerberg is to testify Wednesday before the House Energy and Commerce Committee.

Sen. John Thune, a Republican from South Dakota, told Zuckerberg his company had a 14-year history of apologizing for "ill-advised decisions" related to user privacy. "How is today's apology different?" Thune asked.

"We have made a lot of mistakes in running the company," Zuckerberg responded. "I think it's pretty much impossible, I believe, to start a company in your dorm room and then grow it to be at the scale that we're at now without making some mistakes."

Zuckerberg said Facebook is going through "a broader philosophical shift in how we approach our responsibility as a company".

Senator Orrin Hatch, Republican of Utah, said Zuckerberg's appearance marked the most intense hearing for a tech company since Microsoft, when its founder Bill Gates testified before Congress in March 1998.

Many of the senators' questions seemed to focus on Facebook's basic functions, such as its privacy settings and what it does and doesn't do with user data. Because each of the 44 senators had five minutes to ask questions, there was little time for tough follow-ups.

Associated Press