WASHINGTON — The federal government is stepping up its scrutiny of the world’s biggest tech companies, leaving them vulnerable to new rules and federal lawsuits. Regulators are divvying up antitrust oversight of the Silicon Valley giants and lawmakers are investigating whether they have stifled competition and hurt consumers.

After a spate of unusual negotiations, the Justice Department has agreed to handle potential antitrust investigations related to Apple and Google, while the Federal Trade Commission will take on Facebook and Amazon.

Lawmakers in the House said on Monday that they were looking into the tech giants’ possible anti-competitive behavior. That could lead to the first overhaul of antitrust rules in many decades, an effort to keep up with an industry that didn’t exist when antitrust laws were written.

The question of whether tech companies violate antitrust laws has long been the subject of academic debates and industry griping. But now the industry is in the sights of President Trump, Democrats running for president, Congress and consumers. Silicon Valley has faced fierce criticism over disinformation, privacy breaches and political bias.

Investors pummeled technology stocks on Monday. Shares of Facebook fell more than 7 percent. Google and Amazon shares were also sharply lower, and Apple’s stock fell about 1 percent.

It does not appear that the agencies have opened official investigations. But the scrutiny from Washington could lead to years of headaches for the companies, raising the prospect of lawsuits to break up companies, hefty fines or new laws limiting their reach.

“This is about how do we get competition back in this space,” Representative David Cicilline, Democrat of Rhode Island, said Monday at a news conference. Mr. Cicilline is the chairman of the House Judiciary’s subcommittee on antitrust, which during the next 18 months plans a set of hearings, testimony from executives from top companies, as well as subpoenas for internal corporate documents.

Mr. Cicilline said the investigation would focus on major digital platforms. The House committee on Monday informed four tech companies, Google, Apple, Facebook and Amazon, of the plans. If the House investigation finds wrongdoing, lawmakers will pressure the Justice Department or the F.T.C. to investigate, he said.

“This is long overdue,” he said.

The F.T.C. and Justice Department declined to publicly comment Monday. All four companies also declined to comment, though they have rebutted accusations of anticompetitive behavior in the past.

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