U.S. antitrust enforcers are investigating Exxon Mobil’s plan to acquire Pioneer Natural Resources, which would be the largest oil-and-gas deal in two decades, according to securities filings. 

The Federal Trade Commission has sought additional information from the companies about the deal, a step it takes when reviewing whether a merger could be anticompetitive under U.S. law, Pioneer disclosed in a filing Tuesday. Merger investigations on average take about 10 months to complete, according to data compiled by law firm Dechert.