Updated at 5:30 p.m. with additional comment from Texas Sen. John Cornyn.

WASHINGTON — Facebook CEO Mark Zuckerberg issued a mea culpa in a Senate hearing Tuesday for a massive leak of personal data involving Cambridge Analytica, the voter-targeting firm used by Texas Sen. Ted Cruz and President Donald Trump that's come under fire for improperly acquiring user information to influence voters in the 2016 presidential contest.

But while some senators in both parties pressed Zuckerberg on the scandal, Cruz avoided any mention of Cambridge, instead pushing allegations of political bias at Facebook — and drawing accusations from Democrats that he was protecting the billionaire GOP donor behind the political firm.

"There are a great many Americans who I think are deeply concerned that Facebook and other tech companies are engaged in a pervasive pattern of bias and political censorship," Cruz said, citing examples of alleged suppression of conservative voices, including labeling the Facebook page of pro-Trump social media personalities "Diamond" and "Silk," sisters from North Carolina, "unsafe."

Cruz, who faces Democratic U.S. Rep. Beto O'Rourke in a Senate race in the fall, also asked Zuckerberg if Facebook tracks the political leanings of its employees.

The CEO, appearing in the first of two congressional hearings this week, said that while Facebook's goal is not to engage in political speech, he understands the "concern" over bias, as Facebook and other tech companies are based in the left-leaning Silicon Valley.

Zuckerberg said he tries to "root out" bias in the company. And he rejected a suggestion by Cruz that Palmer Luckey, the wealthy founder of a virtual-reality company acquired by Facebook, was ousted last year because of his conservative political views. Luckey has hosted a Cruz fundraiser in California, according to The Los Angeles Times. He also reportedly supported the pro-Trump group Nimble America, according to multiple news outlets.

"There are a great many Americans who I think are deeply concerned that Facebook and other tech companies are engaged in a pervasive pattern of bias and political censorship," Sen. Ted Cruz, R-Texas, said Tuesday in his questioning of Facebook CEO Mark Zuckerberg. (Andrew Harnik / The Associated Press)

Democrats pounce

Texas Democrats pounced on Cruz following his exchange with Zuckerberg for avoiding asking about Cambridge, a firm largely controlled by GOP billionaire financier Robert Mercer and his daughter, Rebekah. The Mercers supported Cruz's presidential run before backing Trump.

"Texans need someone in their corner as their personal information is stolen, but Cruz would rather crawl over broken glass to protect his billionaire donors like Robert Mercer," the Texas Democratic Party's deputy executive director, Manny Garcia, said in a statement. "Cruz talked tough but didn't utter a word about his deep ties with Cambridge Analytica and how he spent millions to exploit users' personal data in a failed attempt to get you to like him. Just like Donald Trump, Ted Cruz has violated the public trust."

On Twitter, O'Rourke shared a story about how Cruz used the firm during his presidential contest, but he didn't weigh in on the hearing.

Cambridge Analytica sent staff to Cruz campaign headquarters in Houston. The company ultimately did more than $5.8 million worth of work for his campaign.https://t.co/U2HoYMQ36d — Beto O'Rourke (@BetoORourke) April 10, 2018

A spokeswoman for Cruz declined to comment on Texas Democrats' accusations.

The spokeswoman, Catherine Frazier, has previously said that when the first reports emerged in late 2015 that the firm was misusing Facebook data, Cambridge “assured” the Cruz campaign that the allegations were false.

What’s more, the Cruz campaign has said that, in the end, it didn't use the Cambridge data and instead relied on its own.

The Cruz presidential campaign paid Cambridge Analytica $5.8 million between July 2015 and June 2016 for services that included "voter ID targeting," "voter modeling" and "survey research/donor modeling," according to federal election reports. Federal records show no sign of ties between any Cruz campaign and Cambridge since mid-2016.

How it happened

Cambridge, according to Facebook, acquired its data through a University of Cambridge psychology lecturer who created a personality prediction app called “This Is Your Digital Life” that purportedly was for academic research.

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About 270,000 people downloaded the app, unaware that it would be used by a third party to harvest data on their friend networks and that that data would be transferred and used in political campaigns.

Zuckerberg testified that when Facebook learned Cambridge had acquired the data through a third-party app developer, it "demanded" that both the developer and Cambridge delete the data.

Both parties said they did, according to Zuckerberg, who said it was "clearly a mistake to believe them."

Pressed by Florida Sen. Bill Nelson, the top Democrat on the Senate Commerce Committee, about why the tech company hadn't notified its users or the Federal Trade Commission about the issue at the time, the tech CEO said Facebook had wrongfully considered it a “closed case."

Zuckerberg says, in retrospect, it was a mistake to believe Cambridge Analytica and the app developer when they said they deleted the ill-gotten data. Pressed by @NelsonForSenate admits they didnt notify affected FB users b/c they considered it a closed case. — Katie Leslie (@katieleslienews) April 10, 2018

It’s estimated that as many as 87 million users had their data used, up from initial estimates of 50 million.

Facebook executives have embarked on an apology tour in recent days and have begun alerting users whose data may have been improperly shared.

Working with Mueller

Cambridge was far from the only hot topic at the hearing, which stretched on for hours as 44 senators from two committees took turns grilling the Facebook leader.

Zuckerberg disclosed that Facebook is “working with” special counsel Robert Mueller in the federal probe of Russian interference in the 2016 election, and apologized for his company’s failure to prevent the explosion of fake news and disinformation used by the Kremlin to meddle in U.S. politics.

"We didn't take a broad enough view of our responsibility, and that was a big mistake," he said. "And it was my mistake, and I'm sorry. I started Facebook, I run it, and I'm responsible for what happens here."

Facebook is going through “a broader philosophical shift in how we approach our responsibility as a company,” he said, and needs to take a “more proactive role” that includes ensuring that the tools it creates are used in “good and healthy” ways.

He outlined steps the company had taken to restrict outsiders' access to people's personal information. He also said the company was investigating every app that had access to a large amount of information before the company moved to prevent such access in 2014 — actions that came too late in the Cambridge Analytica case.

Zuckerberg also appeared open to some measure of regulation, “if it’s the right regulation,” and pledged to work with lawmakers on what that could entail.

Sen. Graham: Do you embrace regulation?

Zuckerberg: I think the real question as the internet becomes more important in people's lives is, what is the right regulation?

LG: But do you, as a company, welcome regulation?

MZ: I think if it's the right regulation, then yes. pic.twitter.com/xv1SGABAd5 — CBS News (@CBSNews) April 10, 2018

Some Democrats have argued that federal laws might be necessary to ensure user privacy.

Republicans have yet to get behind any such legislation, but that could change.

Texas Sen. John Cornyn, a member of the Judiciary panel and the No. 2 Republican in his chamber, said during a speech ahead of Tuesday's hearing that Facebook's apologies were "not enough" and that legislation could eventually be needed to give consumers more control over their own data privacy.

"This is a serious matter, and I think people expect us to take action," Cornyn told reporters later.

During the hearing, Cornyn asked Zuckerberg a series of questions about how Facebook manages user data and privacy in general and whether it believes it bears any responsibility for its content.

"I agree that we're responsible for the content," Zuckerberg replied.

Zuckerberg is scheduled to testify before House lawmakers on Wednesday.

The Associated Press contributed to this report.