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, tweeted Saturday. "It's clear these platforms can't police themselves ... They say 'trust us.' Mark Zuckerberg needs to testify before Senate Judiciary."

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

On Friday evening, the Menlo Park social media giant -- 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.

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

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, the Associated Press reported. 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.

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?"