The stock market has reacted badly to reports in The Wall Street Journal that two of the nation's largest technology companies—Facebook and Google—are likely to face intensifying antitrust scrutiny from federal regulators in the United States.

As I write this on Monday afternoon, Facebook stock is down 7 percent, while Google stock is down 6.5 percent. The S&P 500 index of large stocks is down less than 1 percent.

An unusual legal arrangement gives the Justice Department and the Federal Trade Commission joint responsibility for antitrust enforcement. The two agencies negotiate to decide which one will represent the government in any particular inquiry.

On Monday, The Wall Street Journal reported that the two agencies had agreed that the Justice Department would investigate Google, while the FTC would focus on Facebook. The Journal had already reported that the Justice Department was preparing a new Google investigation.

The Journal notes that the FTC has already been investigating Facebook on privacy issues but that it "isn't known if the FTC has near-term plans to launch a formal antitrust investigation of Facebook." However, the fact that the FTC obtained Justice Department approval to take the lead on Facebook antitrust issues certainly suggests the agency is taking the issue seriously.

Google and Facebook are big companies with many products and technologies, so an antitrust investigation could go in many different directions. Competitors have long argued that Google wrongly uses its dominant search engine to give itself an unfair advantage in other markets. For example, Yelp has argued that Google formats results in a way that gives Google's own user-reviews service more prominent placement in search results than alternatives like Yelp and TripAdvisor.

As for Facebook, some critics like Sen. Elizabeth Warren (D-Mass.) have argued that antitrust regulators made a mistake in allowing the social media giant to acquire Instagram and WhatsApp. If these companies had remained independent, they could have become formidable Facebook rivals, and some have advocated that US regulators force Facebook to spin them off.

These are just two examples of the many issues that could come up in antitrust investigations. The European Union has already fined Google three times over competition issues, including a $5 billion fine for using the dominance of Android to promote the use of Google's search engine on mobile devices. And there may be a lot more issues still to be explored.

Update: Reuters reports that Apple and Amazon were also mentioned in the accord between the agencies, with the Justice Department taking the lead on investigations of Apple, while the FTC will oversee Amazon. We don't know how soon the agencies might begin investigating these companies.