U.S. federal energy regulators on Monday ordered BP Plc to show cause why a unit of the British oil company should not be found to have manipulated the natural gas market and to pay a fine of $28 million and disgorge $800,000 plus interest.

The Federal Energy Regulatory Commission's Office of Enforcement alleged BP manipulated the next-day, fixed-price gas market at Houston Ship Channel from mid-September 2008 through November of that year.