The U.S. Justice Department is investigating the possibility that airlines have been secretly colluding in order to keep ticket prices high, The Associated Press has learned. Justice Department spokeswoman Emily Pierce confirmed the report, saying that the antitrust investigation seeks to uncover any "unlawful coordination" among some airlines. Although Pierce would not name the companies under investigation, some airline stocks have already begun to drop in the wake of the report. Mergers have left American Airlines, Delta Air Lines, Southwest Airlines, and United Airlines in control of 80 percent of the seats available in the United States, AP says.