Stocks tumbled on Monday, dragged down by shares of technology companies, which until recently had propelled the nine-year bull market ever higher.

Facebook was at the heart of the sell-off. Its shares closed down 6.8 percent after news emerged over the weekend that a political data firm with links to President Trump’s 2016 campaign had harvested private information from more than 50 million Facebook profiles.

The news reports could open the door to greater government scrutiny and potential regulation of the technology sector. Already, government officials in the United States, Europe and elsewhere have been demanding tougher oversight of the world’s largest tech companies. That, in turn, could erode the industry’s profits and potentially force some companies to adjust their business models.

Facebook’s decline on Monday was the stock’s worst single-day fall since 2014, and it weighed on the other tech giants. Google’s parent company, Alphabet, fell more than 3 percent. Amazon and Microsoft dropped more than 1.7 percent. And Apple, the largest American company by market capitalization, sank 1.5 percent.