RALEIGH, N.C. — North Carolina utilities regulators are hiring a former federal prosecutor and the Chicago law firm he heads to dig into whether they were misled ahead of the takeover creating America's largest electric company.

The North Carolina Utilities Commission said Wednesday it has hired Anton Valukas and the Jenner & Block law firm to investigate what happened before regulators approved Charlotte-based Duke Energy Corp. taking over Raleigh-based Progress Energy Inc.

The companies completed their merger last month, days after North Carolina regulators approved. Hours after the deal was done, Duke Energy's board ousted Progress Chief Executive Bill Johnson as the new company's president and CEO, replacing him with Jim Rogers, Duke's president and CEO, who was to be chairman of the combined utility.

Valukas was the court-appointed examiner in the Lehman Brothers bankruptcy case. He found the investment bank hid $50 billion in debt before its 2008 collapse, which created a crisis that threatened the world's financial system.

Duke announced Wednesday that it has asked the Utilities Commission and the Public Service Commission of South Carolina to lower residential electric rates, starting Sept. 1, to help the company deliver its promise of saving consumers $650 million through the merger.

If the rates are approved, the average monthly bill for a Progress customer in North Carolina would go down 85 cents, to $105.15. The average bill for a Duke customer in the state would drop 92 cents, to $104.94.

"We are scheduling and operating our generation plants as a combined fleet to obtain maximum efficiency, and using the expertise and best practices of the combined company to lower fuel costs for our customers," Keith Trent, executive vice president for regulated utilities, said in a statement.

Company officials said the lower costs would save consumers in the two states about $70 million over the next 12 months.