The United States Justice Department said on Tuesday it was opening a broad investigation of major digital technology firms into whether they engage in anti-competitive practices, the strongest sign the Trump administration is stepping up its scrutiny of Big Tech.

The review will look into “whether and how market-leading online platforms have achieved market power and are engaging in practices that have reduced competition, stifled innovation, or otherwise harmed consumers,” the Justice Department said in a statement.

The Justice Department did not identify specific companies but said the review would consider concerns raised about “search, social media, and some retail services online” – an apparent reference to Alphabet Inc, Amazon.com Inc and Facebook Inc, and potentially Apple Inc.

A Justice Department spokesman declined to provide a list of companies that would be scrutinised.

Google and Apple declined to comment, referring to prior statements by executives, while Facebook and Amazon did not immediately comment.

Facebook fell 1.7 percent in after-hours trading, while Alphabet fell 1.0 percent, Amazon was down 1.2 percent and Apple was 0.4 percent lower.

The announcement comes a day before the Federal Trade Commission is set to announce a $5bn penalty to Facebook for failing to properly protect user privacy.

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