The Federal Trade Commission, a key congressional committee and state prosecutors each announced inquiries Monday into Facebook’s privacy practices, leaving the internet giant scrambling to contain a growing scandal over how it treats its users.

The FTC confirmed that it is investigating whether release of data on more than 50 million Facebook users — information that ended up in the hands of a British political consultancy tied to the Trump campaign — breaches a 2012 privacy settlement Facebook reached with the U.S. government to better protect data.

On Capitol Hill, Sen. Charles E. Grassley, Iowa Republican and chairman of the Judiciary Committee, called on Facebook founder Mark Zuckerberg to explain his company’s actions at an April 10 hearing.

More than three dozen state attorneys general fired off a letter asking Mr. Zuckerberg for more details about the 50 million users’ data.

“Our trust has been broken,” the attorneys general said in their letter, which suggests more legal questions lie ahead.

International regulators are also on the case. Mr. Zuckerberg has been called to deliver testimony to the British Parliament, and Germany’s justice minister told the company to expect more rules governing how it handles users’ data.

The company’s practices have come under scrutiny after news reports detailed the compilation of users’ data by a British university professor. The information was shared with Cambridge Analytica, a political consulting firm that worked for the Trump campaign in 2016.

Facebook says the professor wasn’t supposed to share the data and that the information was improperly “leaked.”

Mr. Zuckerberg, in advertisements in American newspapers this week, acknowledged a “breach of trust” and promised to take steps to prevent a repeat.

“We have a responsibility to protect your information. If we can’t, we don’t deserve it,” the company founder said in the signed, full-page ads.

He said Facebook already has taken steps to limit data flowing to outsiders and promised a review of all apps that might have been able to access data before the fixes.

Facebook’s stock, which took a beating last week after the data scandal was exposed, slid in early trading Monday after the FTC’s announcement but returned to about even by the end of the day.

The company said it will answer the questions it faces from regulators.

“We remain strongly committed to protecting people’s information. We appreciate the opportunity to answer questions the FTC may have,” said Rob Sherman, Facebook’s deputy chief privacy officer.

The company did not comment specifically on the 2012 settlement it reached with the FTC after an earlier round of accusations of having duped users over handling of their data. That settlement required Facebook to be more honest with users about their data and its uses.

The FTC said compliance with that settlement will be a focus of the investigation.

“The FTC takes very seriously recent press reports raising substantial concerns about the privacy practices of Facebook. Today, the FTC is confirming that it has an open non-public investigation into these practices,” Tom Pahl, acting director of the agency’s consumer protection bureau, said in a statement.

Several congressional committees have said they want to hear about Facebook’s handling of data and how Cambridge Analytica ended up with information on 50 million users.

Facebook said the data were supposed to have been shared only with an academic researcher, Aleksandr Kogan, a professor at Cambridge University, but was “leaked” to Cambridge Analytica.

Mr. Kogan has said both companies are making him the scapegoat.

Facebook’s explanations are being challenged by prosecutors in Cook County, Illinois, where State’s Attorney Kimberly M. Foxx filed a lawsuit accusing Facebook, Cambridge Analytica and its parent company of breaking the state’s law against consumer fraud.

Ms. Foxx said Facebook “not only allowed but encouraged” the behavior.

“Though it may have started as a social network, Facebook’s business model has shifted over the years into what is now one of the biggest data-mining companies in the world,” the lawsuit says. “Facebook now uses its platform … to manipulate users into making the decisions that Facebook and its business partners want them to make.”

Facebook defended itself Monday against accusations that it was tracking people’s calls and text messages from their smartphones, even when those communications were unrelated to Facebook.

The company said it was collecting the data, but only after users gave permission through an “opt-in feature.” It worked only on Android phones.

Facebook said it doesn’t sell the data and doesn’t collect the content of the messages or calls.

“You are always in control of the information you share with Facebook,” the company said.

But users seemed stunned to hear what they had opted into by clicking a certain setting when they loaded a Facebook-related app on their phone.

Facebook allows users to download all of the data the company has on them, and several people posted snapshots of their files online, detailing huge tranches of data including detailed call logs and intimate photos.

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