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.