A group of Democratic lawmakers in the United States urged the Department of Justice in a letter today, Tuesday, to investigate "potential conspiracies that violate antitrust rules" between American oil producers and the Organization of the Petroleum Exporting Countries (OPEC). Last month, the Federal Trade Commission gave the green light for Exxon Mobil to proceed with the acquisition of Pioneer Natural Resources in a deal worth $60 billion, but it prohibited former Pioneer CEO Scott Sheffield from joining the Exxon board amid allegations that he attempted to collude with OPEC to raise oil prices.