NEW YORK (CNN/Money) - Utility stocks tumbled Wednesday after the U.S. Federal Energy Regulatory Commission ordered four big utility companies to comply with its investigation into the possible manipulation of power prices in California or have their authority to issue market-based rates revoked.

The companies named in the order are Williams Energy Marketing & Trading Co., Portland General Electric Co., El Paso Electric Co. and Avista Corp.

Shares of Avista (AVA: down $2.28 to $11.87, Research, Estimates) and Williams Cos. (WMB: down $2.11 to $9.10, Research, Estimates) fell more than 13 percent Wednesday while El Paso (EE: down $1.05 to $13.40, Research, Estimates) fell more than 6 percent. Portland General Electric is a unit of bankrupt energy trader Enron, but is not part of Enron's bankruptcy filing.

The FERC said Tuesday it began investigating in February whether utility companies improperly manipulated power prices in the West, but the firms named in the order "are not fully cooperating with the investigation, thus hindering the commission's ability to determine whether...rates charged may have been unjust and unreasonable."

The commission is examining whether more than 100 energy companies' trading strategies contributed to soaring electricity prices -- and blackouts -- in California in late 2000 and early 2001. The state has demanded refunds of nearly $9 billion, claiming that power companies manipulated the market to increase profits.

Electricity prices jumped tenfold during that time, bankrupting a major utility in the state and leaving thousands of Californians in the dark.