WASHINGTON — Fending off accusations of stifling competition, four Big Tech CEOs — Facebook’s Mark Zuckerberg, Amazon’s Jeff Bezos, Sundar Pichai of Google and Tim Cook of Apple — answered for their companies’ practices before Congress as a House panel caps its yearlong investigation of market dominance in the industry.

The powerful CEOs sought to defend their companies amid intense grilling by lawmakers on Wednesday.

The executives provided bursts of data showing how competitive their markets are, and the value of their innovation and essential services to consumers. But they sometimes struggled to answer pointed questions about their business practices. They also confronted a range of other concerns about alleged political bias, their effect on U.S. democracy and their role in China.

The four CEOs were testifying remotely to lawmakers, most of whom were sitting, in masks, inside the hearing room in Washington.

Among the toughest questions for Google and Amazon involved accusations that they used their dominant platforms to scoop up data about competitors in a way that gave them an unfair advantage.

Bezos said in his first testimony to Congress that he couldn’t guarantee that the company had not accessed seller data to make competing products, an allegation the company and its executives have previously denied.

Regulators in the U.S. and Europe have scrutinized Amazon’s relationship with the businesses that sell on its site and whether the online shopping giant has been using their data to create its own private-label products.

“We have a policy against using seller specific data to aid our private label business,” Bezos said in a response to a question from U.S Rep. Pramila Jayapal, a Washington Democrat. “But I can’t guarantee to you that that policy hasn’t been violated.”

The companies face legal and political offensives on multiplying fronts, from Congress, the Trump administration, federal and state regulators and European watchdogs.

As Democrats largely focused on market competition, several Republicans aired longstanding grievances that the tech companies are censoring conservative voices and questioned their business activities in China. “Big Tech is out to get conservatives,” said Rep. Jim Jordan, of Ohio.

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