By David Shepardson

WASHINGTON (Reuters) - The leading U.S. consumer protection regulator and attorneys representing 37 states stepped up pressure on Facebook Inc <FB.O> on Monday to explain how the social network allowed data of 50 million users get into the hands of a political consultancy.

The U.S. Federal Trade Commission took the unusual step of announcing that it had opened an investigation into the company - which it generally only does in cases of great public interest - citing media reports that raise what it called "substantial concerns about the privacy practices of Facebook."

On the same day, a bipartisan coalition of 37 state attorneys wrote to Facebook, demanding to know more about the company's role in the manipulation of users' data by the consultancy, Cambridge Analytica, which used it to target U.S. and British voters in close-run elections.

“These revelations raise many serious questions concerning Facebook’s policies and practices, and the processes in place to ensure they are followed,” the letter said. "We need to know that users can trust Facebook. With the information we have now, our trust has been broken.”

Facebook shares fell as much as 6.5 percent, briefly dipping below $150 for the first time since July 2017, before recovering the day's losses to close up 0.4 percent at $160.06.

The shares are still down 13 percent since March 16, when Facebook first acknowledged that user data had been improperly channeled to Cambridge Analytica. The company has lost more than $70 billion in market value since then.

The recovery in Facebook's stock on Monday may have been due to investors taking advantage of the lower stock price and the belief that the latest regulatory scrutiny may not ultimately hurt the company's relative long-term growth prospects, Wall Street analysts said.

The FTC investigation is looking at more than whether Facebook violated a 2011 consent order it reached with the FTC over its privacy practices, a person briefed on the matter told Reuters.

If the FTC finds Facebook violated terms of the consent decree, it has the power to fine it thousands of dollars a day per violation, which could add up to billions of dollars.

“We remain strongly committed to protecting people’s information," Facebook Deputy Chief Privacy Officer Rob Sherman said in a statement on Monday. "We appreciate the opportunity to answer questions the FTC may have.”

ALL APOLOGIES

Lawmakers in the United States and Europe continue to pressure Facebook and Chief Executive Mark Zuckerberg to explain the company's privacy practices.