SPRINGFIELD | Attorney General Lisa Madigan alleged Thursday that corporate manipulation caused the huge jump in electric rates that Illinois consumers are facing, and she asked federal regulators to suspend the rates and investigate.

Electric rates have soared since January, when a 10-year freeze on prices expired. The new prices were set through a "reverse auction" that was meant to keep rates as low as possible.

But Madigan says the auction yielded prices far higher than the actual cost of producing electricity. That's because some 15 power companies manipulated the process, she claimed in a complaint to the Federal Energy Regulatory Commission.

Exelon Corp., the parent of power company ComEd, immediately denied the allegations.

"The charges are false. The Illinois auction has been deemed fair and reasonable by all regulatory agencies," Exelon spokeswoman Kathleen Cantillon said in a statement.

Ameren Corp. said it would not respond until it had a chance to review Madigan's complaint.

The federal commission said it would review Madigan's complaint and any responses from the power companies before deciding whether to proceed.

Last year, the Illinois Commerce Commission oversaw a reverse auction, where power suppliers offered the lowest prices at which they would sell electricity to Ameren and ComEd, which then sell the electricity to consumers.

The auction produced unexpectedly high increases over previous rates -- 22 percent higher on average for ComEd customers and 55 percent higher for some Ameren customers. For some individual customers, the real increases turned out to be even more.

Madigan argued the huge increases must be a result of corporate misconduct. She said the average prices offered by wholesalers -- which include the parent companies of ComEd and Ameren -- are twice the actual costs of producing the electricity.

One such arrangement amounts to "a massive transfer of money from ComEd's captive customers to the stockholders of Exelon Corporation," her complaint said.

The version of Madigan's complaint offers several scenarios for how companies might have manipulated the process.

For instance, Madigan speculates that existing contracts might have had secret provisions that forced some bidders to drop out of the auction, reducing competition. Or companies might have persuaded bidders to drop out by promising to buy their electricity at favorable rates.

The Citizens Utility Board applauded Madigan's actions.

"This auction was a scheme designed by ComEd and Ameren to maximize profits of their parent companies at consumers' expense," said CUB Executive Director David Kolata.