Sempra Energy yesterday disclosed that federal investigators are probing the company regarding the spike in natural gas prices last year that sent costs soaring more than 45 percent from Thanksgiving to mid-December. The disclosure came on the same day that the Williams Cos. announced that it agreed to forgo $140 million to settle allegations of market-rigging during the state's electricity crisis of 2000-01. San Diego-based Sempra said that it has been subpoenaed by the Commodity Futures Trading Commission in an investigation of higher natural gas prices. The increases have raised home heating costs and led some major industrial users of...