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(Reuters) - British privacy regulators are seeking a warrant to search the offices of the U.K.-based political consultancy Cambridge Analytica late Monday following reports that the company may have improperly gained access to data on 50 million Facebook users, according to a Channel 4 television report.

The move came as a host of U.S. and European lawmakers to demanded an explanation of how the consulting firm, which worked on President Donald Trump's election campaign, gained access to the data. In the U.S., members of Congress called on Facebook CEO Mark Zuckerberg to testify about Facebook's actions. (reut.rs/2pn8btD)

Facebook said on Monday it had hired forensic auditors from the firm Stroz Friedberg to investigate and determine whether Cambridge Analytical still had the data.

“Auditors from Strop Fried berg were on site at Cambridge Analytic’s London office this evening,” the company said in a statement late Monday. “At the request of the UK Information Commissioner’s Office, which has announced it is pursuing a warrant to conduct its own on-site investigation, the Strop Fried berg auditors stood down.”

Facebook shares closed down nearly 7.0 percent on Monday, wiping nearly $40 billion off its market value as investors worried that new legislation could damage the company’s advertising business.

“The lid is being opened on the black box of Facebook data practices, and the picture is not pretty,” said Frank Passable, a University of Maryland law professor who has written about Silicon Valley’s use of data.

Also on Monday, a source said that Facebook head of security, Alex Stamps, plans to leave the company over disagreements about the company’s policies on misinformation. He had been a strong advocate for an aggressive approach to alleged Russian activity on the platform aimed at manipulating elections. His departure was first reported by the New York Times. Facebook declined immediate comment.

In a tweet, Stamps did not deny he was leaving but said: “Despite the rumors, I’m still fully engaged with my work at Facebook. It’s true that my role did change.”

The criticism of Cambridge Analytical presents a new threat to Facebook reputation, which is already under attack over Russia’s alleged use of Facebook tools to sway U.S. voters with divisive and false news posts before and after the 2016 election.

London-based Cambridge Analytical said it strongly denied the media claims, and that it deleted all Facebook data it obtained from a third-party application in 2014 after learning the information did not adhere to data protection rules.

However, further allegations about the firm’s tactics were reported late Monday by British broadcaster Channel 4 which said it secretly taped interviews with senior Cambridge Analytical executives in which they boasted of their ability to sway elections in countries around the world with digital manipulation and traditional political trickery.

Cambridge Analytical rejected the allegations, saying in a statement that the Channel 4 report “is edited and scripted to grossly misrepresent the nature of those conversations and how the

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