The US department of justice has launched of a review of whether “market-leading online platforms” are behaving in a way which reduces competition, stifles innovation, or harms consumers.
It did not name specific companies but said the review would consider concerns about “search, social media, and some retail services online” – an apparent reference to the likes of Google owner Alphabet, Amazon, Facebook and possibly Apple.
Shares in the companies fell in after-hours trading on Wall Street.
The DoJ said: “The goal of the department’s review is to assess the competitive conditions in the online marketplace in an objective and fair-minded manner and to ensure Americans have access to free markets in which companies compete on the merits to provide services that users want.”
It comes as, separately, the US Federal Trade Commission reportedly prepares to announce a $5bn (£4bn) fine for Facebook for failing to protect user privacy.
US politicians across the spectrum have expressed growing concerns about the power and size of the big tech firms.
Democratic presidential candidate Elizabeth Warren has called for the break-up of the likes of Amazon, Apple, Google and Facebook.
Donald Trump has also frequently criticised the big tech companies and claimed they are biased against him and conservatives in general.
The firms are also under pressure globally, with Google fined billions after a series of investigations by the European Commission and Amazon also under scrutiny in Brussels.