(Reuters) — Facebook Inc. Chief Executive Mark Zuckerberg plans to testify before Congress, a source briefed on the matter said on Tuesday, as he bows to pressure from lawmakers insisting he explain how 50 million users’ data ended up in the hands of a political consultancy.

Lawmakers in the United States and Europe are demanding to know more about the company’s role in consultancy Cambridge Analytica’s use of the data to target U.S. and British voters in close-run elections.

Facebook said on Tuesday the company had received invitations to testify before Congress and that they were talking to legislators.

House Energy and Commerce Committee spokeswoman Elena Hernandez said: “The committee is continuing to work with Facebook to determine a day and time for Mr. Zuckerberg to testify.”

On the same day, Mr. Zuckerberg turned down British lawmakers’ invitations to explain to a British parliamentary committee what went wrong.

The company said it would instead send one of his deputies, suggesting that Chief Technology Officer Mike Schroepfer or Chief Product Officer Chris Cox had the expertise to answer questions on the complex subject.

The head of the committee called Mr. Zuckerberg’s decision “astonishing” and urged him to think again.

Facebook shares were down 3.3% on Tuesday and have fallen almost 17% since March 16, when Facebook first acknowledged that user data had been improperly channeled to Cambridge Analytica, which was hired by President Donald Trump’s 2016 presidential campaign.

The revelations have raised investor concerns that any failure by big tech companies to protect privacy could deter advertisers and lead to tougher regulation.

It was unclear when or before which committee Mr. Zuckerberg would testify.

The U.S. Senate Judiciary Committee said on Monday it had invited Mr. Zuckerberg, as well as the CEOs of Alphabet Inc. and Twitter Inc., to testify at an April 10 hearing on data privacy.

The House Energy and Commerce Committee and Senate Commerce Committee had already formally asked Mr. Zuckerberg to appear at a congressional hearing.

The Federal Trade Commission took the unusual step of announcing on Monday 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.”