WASHINGTON - Facebook chief executive Mark Zuckerberg said that his company is working with special counsel Robert Mueller in the sweeping Russia investigation, during his first appearance before Congress Tuesday over a data breach scandal.

Zuckerberg, during a hours-long grilling, told 44 US senators that Facebook is working with Mueller's team, but stressed that he wants to be careful because the "work with the special counsel is confidential."

The congressional hearing came nearly a month after news broke that Cambridge Analytica, a data consulting firm with ties to President Donald Trump's campaign, accessed information from as many as 87 million Facebook users without their knowledge.

Russian groups were alleged to have taken advantage of the social media platform to interfere with the 2016 US presidential election.

Facebook was "too slow to spot and respond to Russian interference," Zuckerberg said. "It's clear now that we didn't do enough to prevent these tools from being used for harm."

"And that goes for fake news, foreign interference in elections and hate speech, as well as developers and data privacy," the 33-year-old CEO added, in an apologetic tone, noting that Facebook plans to have more than 20,000 employees working in security and content review by the end of this year.

He also revealed that Facebook is developing artificial intelligence technology in order for hate speech to be "flagged upfront" in five or 10 years.

Besides, Facebook said it has dramatically restricted the amount of data that developers can access and proactively reviews the apps on its platform. But several Senators have expressed their dismay over its actions or lack of action.

"In the past, many of my colleagues on both sides of the aisle have been willing to defer to tech companies' efforts to regulate themselves," said Senate Commerce Chairman John Thune. "But this may be changing."

Zuckerberg admitted that Facebook recognizes the need to "take a more proactive role."

Facebook's stock price ended up 4.5 percent for the day, rallying to its highest point during the testimony, after the data scandal had wiped away tens of billions of dollars from Facebook's market value.

Zuckerberg, who traded his usual gray T-shirt and jeans attire for a dark suit on Tuesday, is scheduled to appear Wednesday before the House Energy and Commerce Committee.