Facebook Chief Executive Officer Mark Zuckerberg was questioned by officials from the U.S. Federal Trade Commission over two days this week as part of an antitrust investigation into the social media company, according to a person familiar with the matter.

The FTC’s probe into the company was disclosed last summer. The agency is one of multiple enforcers, including the U.S. Justice Department and states attorneys general, exploring Facebook’s market power and influence.

“We are committed to cooperating with the U.S. Federal Trade Commission’s inquiry and answering the questions the agency may have,” a Facebook spokesman said in a statement. A spokeswoman for the FTC declined to comment.

Facebook is under investigation for whether it’s abusing its outsized share of the online advertising market, in addition to whether its acquisitions of Instagram and WhatsApp and its copying of competing apps’ features violate antitrust laws. At a congressional hearing last month, alongside the CEOs of three other technology giants, Zuckerberg defended those deals and argued that Facebook’s numerous products have a lot of competition.

Zuckerberg’s FTC testimony was reported earlier by Politico.

The FTC was criticized earlier this month for failing to meet with Zuckerberg during a separate, privacy-related investigation that started in 2018. “Sometimes it’s important to depose the CEO and sometimes it’s not necessary, but where it’s important and helpful, we try to do it,” FTC Chairman Joe Simons told a Senate panel on Aug. 5.

Democratic Senator Richard Blumenthal, who criticized the agency for its failure to depose Zuckerberg in its Cambridge Analytica data-protection probe, which resulted in a record $5 billion fine, responded that “the credibility of this investigation is going to depend on its completeness and aggressiveness.”

“It seems to me that the public will be satisfied only if you do depose the very top executives,” Blumenthal added.