WASHINGTON, Ramadan 15, 1436, July 02, 2015, SPA -- The U.S. Department of Justice on Wednesday confirmed it is investigating multiple major airlines for possibly coordinating with each other to fix prices, according to UPI. DOJ spokeswoman Emily Pierce told CNN the department sent subpoenas to a number of airlines Tuesday. Officials are investigating "possible unlawful coordination" between the unidentified airlines to limit the number of available seats and flights to keep prices high. United Continental Holdings Inc. told The Wall Street Journal that United Airlines received a letter and is cooperating with the investigation. A source familiar with the investigation told the Journal the probe has been ongoing for a number of weeks. U.S. Sen. Richard Blumenthal, D-Conn., reportedly asked the DOJ to investigate the airlines. Blumenthal sent a letter to the department after an international airline conference last month in Miami where executives allegedly said they were trying to protect profit margins by limiting service while they had cheap fuel. The senator said he wanted the DOJ to "investigate this apparent anti-competitive conduct potentially reflecting a misuse of market power, and excessive consolidation in the airline industry." Within hours after the news broke, airline stocks began to plummet. American Airlines fell 2.84 percent, though at one point it was down 4.1 percent. United Airlines was down 2.49 percent; Delta Airlines was down 1.97 percent; Southwest Airlines was down 1.42 percent; and JetBlue Airways was down 3.37 percent. --SPA 00:42 LOCAL TIME 21:42 GMT www.spa.gov.sa/w