A group of traders has sued exchange giant CME Group, alleging that its policies favored high-frequency traders and defrauded other investors. Three traders filed the complaint Friday in federal court in Illinois, accusing CME and its subsidiary the Chicago Board of Trade of falsely labeling as "real-time" price data that was sold to "the entire financial world." In reality, the complaint argues, CME charged high-frequency traders to see key data and trade upon that information before the rest of the market. The exchanges "engaged in a fraud on the marketplace, deceptive practice and failed to maintain a marketplace that is free from market disruption and market manipulation," the complaint said. The complaint covers the period of 2007 through April 10, 2014, and concerns a range of equity, debt, commodities contracts and derivatives. The Chicago-based CME, one of the world's leading exchange operators, dismissed the suit as "without merit" and vowed to defend itself "vigorously." "The suit is devoid of any facts supporting the allegations and, even worse, demonstrates a fundamental misunderstanding of how our markets operate," CME said in a statement. "It is sad when plaintiffs' lawyers bring a suit based on a desire for publicity, and in the rush to file a suit fail to undertake even the most basic effort to determine if there is any basis for their allegations." High-frequency traders employ algorithmic formulas and whip-speed transmission lines to trade a large number of orders at split-second speeds. The practice is legal. Proponents argue that high-speed trading provides added liquidity to the markets. Critics say it manipulates the market to win commissions and guaranteed profits at the expense of average investors. A number of official investigations have been launched into the controversial practice, including by the Justice Department, the US Securities and Exchange Commission, the Commodities Futures Trading Commission and the New York state attorney general. Public debate over high-speed trading heated up with the March 31 release of a book by prominent author Michael Lewis. In "Flash Boys: A Wall Street Revolt," Lewis argues that stock exchanges, large Wall Street banks and high-speed traders have together established a system that results in higher prices for the stock purchases of ordinary investors.