The federal government may be warming up its antitrust enforcement machine and pointing it at big tech. On Monday, the House Judiciary Committee announced a sweeping antitrust probe of unspecified technology companies. In a statement, it promised “a top-to-bottom review of the market power held by giant tech platforms,” which would be the first such Congress has ever undertaken.

Shares of Facebook, Google, Amazon, and Apple dropped significantly Monday after published reports suggested that federal authorities are preparing for investigations into anticompetitive behavior by several of these technology giants.

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“We’re seeing a lot of concern that tech companies have just gotten too big, that they control too much of what they do, that they’re stomping on rivals and that they’re making it so that consumers only have one choice,” said Associated Press technology reporter Rachel Lerman.

Some of the underlying developments appear to represent a divvying up of turf between the Department of Justice’s antitrust agents and the Federal Trade Commission, which also holds antitrust authority.

The Justice Department would reportedly hold authority over Google and Apple, while the FTC would take point on investigations of Facebook and Amazon.

Over the weekend, multiple reports said the Justice Department was preparing a competition investigation into Google.

On Monday, June 3, the Wall Street Journal cited unnamed sources to report that the FTC had secured the rights to bring a possible investigation into Facebook.

Rachel Lerman said, “Eventually if the we get to a point where the Justice Department or the Federal Trade Commission decides to actually break up these companies, which is a pretty significant decision, then consumers might see the way that they use products will change.

Investors may have reacted immediately to the uncertainty, but investigations—if any materialize—would take years.

Pressure has been mounting on government to scrutinize the companies for some time, as backlash against tech companies’ reach and power grow among consumers and politicians.

European authorities have covered ground U.S. regulators have barely considered, resulting in billions of dollars in fines for Google, and lesser fines for other companies.

The FTC is already investigating Facebook for possible privacy violations. The FTC declined to comment and Facebook did not immediately respond to a message for comment. Facebook has set aside $3 billion for a possible fine for that investigation and said it could be as high as $5 billion.