MENLO PARK — A U.S. senator is demanding that Facebook CEO Mark Zuckerberg testify before the Senate Judiciary Committee, after reports that a company employed by President Donald Trump’s 2016 campaign accessed profile data from more than 50 million Facebook users without their permission.

“This is a major breach that must be investigated,” Sen. Amy Klobuchar, a Minnesota Democrat and member of the committee, tweeted Saturday. “It’s clear these platforms can’t police themselves … They say ‘trust us.’ Mark Zuckerberg needs to testify before Senate Judiciary.”

The Judiciary Committee oversees the Department of Justice and examines proposed legislation. Klobuchar is ranking member of the subcommittee on antitrust, competition policy and consumer rights. The committee, Republican led, has subpoena power.

Facebook did not immediately respond to a request for comment on the senator’s demand.

Furor over the Menlo Park social media giant’s leak comes as major Silicon Valley tech firms face increasing calls for their regulation, driven by concerns about market dominance, use of their platforms by Russian trolls for election meddling, and extremist, exploitative and offensive content. Klobuchar, with fellow Democratic Sen. Mark Warner and Republican Sen. John McCain, has proposed a bill called the Honest Ads Act to force large tech companies such as Facebook, Google and Twitter to make public certain information about political ads on their platforms.

Europe in May will begin imposing strict new rules on the processing and movement of personal data, mandating “unambiguous” consent by consumers and allowing fines up to 4 percent of a company’s global annual revenue.

Cambridge Analytica harvested profile information from more than 50 million Facebook users without their permission, the Associated Press reported.

On Friday evening, Facebook — with the New York Times poised to publish on Saturday an article about the massive data leak — admitted in a news release that Cambridge Analytica had years ago obtained user data from a “personality prediction” app that was downloaded by about 270,000 people.

The app developer could then access “information such as the city they set on their profile, or content they had liked, as well as more limited information about friends who had their privacy settings set to allow it,” Facebook said.

The developer accessed the data through what were the proper channels at the time, but “lied to us and violated our Platform Policies by passing data (to) Cambridge Analytica,” the company said.

Facebook subsequently changed the rules governing developers’ use of its data, and now requires them to justify and explain proposed data collection before they’re allowed to access user information or ask for it, the company said.

Cambridge Analytica denied wrongdoing, saying it had deleted the data it received from the developer. Facebook, however, said in its news release that it had received reports several days ago that not all the data was deleted.

“We are moving aggressively to determine the accuracy of these claims,” the company said.

Cambridge Analytica, linked to Trump campaign strategist Steve Bannon, used the data to develop techniques that formed the foundation of its work on the Trump campaign, The New York Times and The Guardian reported.

Cambridge said in a statement none of the data it received from the developer was used for services provided to the Trump campaign.

“Cambridge Analytica only receives​ and use​s​ data that has been obtained legally and fairly,” said the company, adding that it had deleted the developer’s data when it “became clear” that it broke Facebook’s terms of service.

Facebook executives took to Twitter on Saturday to argue strenuously that the data leak was not a data breach.

“This was unequivocally not a data breach,” tweeted longtime executive Andrew Bosworth. “People chose to share their data with third party apps and if those third party apps did not follow the data agreements with us/users it is a violation. No systems were infiltrated, no passwords or information were stolen or hacked.”

One of the New York Times reporters who wrote the newspaper’s story on the leak responded. “Facebook officials today playing semantic — but legally very important to regulators — word games about a data ‘breach,'” Nick Confessore‏ tweeted. “But who needs to steal passwords when Facebook will just give some dude access to your profile and not even check his app out that closely?”