British privacy regulators are seeking a warrant to search the offices of the 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 U.S. and European lawmakers 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.

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

“Auditors from Stroz Friedberg were on site at Cambridge Analytica’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 Stroz Friedberg 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 Pasquale, 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 Stamos, 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.

(Reuters)