The U.S. Justice Department has joined the search for an estimated $600 million missing from MF Global, CNN reported, citing sources close to the investigation. Federal prosecutors and the FBI were working along with investigators from the U.S. Securities and Exchange Commission and the Commodities Futures Trading Commission to try to locate the funds, which were unaccounted for amid the brokerage's Chapter 11 bankruptcy proceedings. Earlier fears were that around $1 billion had vanished from the coffers of MF Global, which took a major blow when the European bond market went south. The company, which is run by former Goldman Sachs Chief Executive Officer and New Jersey Gov. Jon Corzine, filed for bankruptcy protection Monday. The discovery of the discrepancy scuttled an 11th-hour attempt by MF Global leadership to arrange a sale to Interactive Brokers. Authorities now want to know if the firm improperly dipped into clients' funds to cover its costs as the bond losses mounted, CNN said. Trading firms are required by law to keep customer funds separate from company assets. The FBI would not confirm or deny its role in the investigation.