The U.S. Department of Justice on Tuesday announced the opening of an investigation into the power and practices of market-leading online platforms.

The probe aligns with a plan Attorney General William Barr alluded to during his confirmation hearing in January.

According to the DOJ, the review “will consider the widespread concerns that consumers, businesses, and entrepreneurs have expressed about search, social media, and some retail services online.” Names of individual companies are not mentioned in the release, but the belief is Facebook, Google, Amazon, and Apple will be among those in the crosshairs.

“Without the discipline of meaningful market-based competition, digital platforms may act in ways that are not responsive to consumer demands,” said Assistant Attorney General Makan Delrahim of the Antitrust Division in a statement. “The Department’s antitrust review will explore these important issues.”

At his confirmation, Barr expressed concerns over the growth of large technology companies and wondered aloud how the industry’s giants got that way without interference from the Antitrust Division. He said he was “interested in stepping back and reassessing or learning more about how the Antitrust Division has been functioning and what their priorities are.”

“I don’t think big is necessarily bad,” Barr said, “but I think a lot of people wonder how such huge behemoths that now exist in Silicon Valley have taken shape under the nose of antitrust enforcers.”

Big Tech has been under fire from politicians across the aisle in recent months. Sen. Elizabeth Warren (D-Mass.), a candidate for president, got the ball rolling with a pitch to break up world-leading technology companies in March, and others, including Sens. Amy Klobuchar (D-Minn.) and Ted Cruz (R-Texas), have also been outspoken critics.

Klobuchar, another presidential candidate and a member of the Senate Judiciary Subcommittee on Antitrust, Competition Policy and Consumer Rights, released a statement regarding the DOJ’s probe. “The American people deserve to know whether these tech giants are unlawfully stifling competition and how our laws and enforcers can encourage innovation while protecting consumers,” she said. “I will work to ensure that this review and any resulting investigations are thorough, public, and result in action that actually helps consumers.”