In Depth: Blackout Additional information about the energy crisis in California and its effects on the nation from The New York Times and the PBS series "Frontline" is available at www.pbs.org "Blackout," a documentary co-produced by Frontline and The New York Times, aired on June 5.

WASHINGTON, June 29 - An investigative arm of Congress faulted federal regulators today for claiming that there was no evidence to support accusations that power companies manipulated California's electricity market.

The General Accounting Office reviewed a study by the Federal Energy Regulatory Commission issued in February. The energy agency found no evidence that companies withheld power to drive up prices.

Politicians in California and some economists have been asserting for months that power companies withheld available electrical generating capacity to create or worsen shortages. Generating companies had cited the energy agency's report to rebut those accusations.

The accounting office said the agency's work looked only at physical reasons for outages and did not explore the possibility that generating companies used bidding strategies to withhold supplies. Moreover, the office found that the study did not prove that cutoffs had occurred for unavoidable physical reasons.

"FERC's study was not thorough enough to support its overall conclusion that audited companies were not physically withholding electricity supply to influence prices," the accounting office said.

The office said that industry experts it consulted agreed it was "practically impossible" to determine whether cutoffs were justified merely by examining physical evidence because plants frequently ran with some physical problems.

The accounting office's study comes a week after three former employees of Duke Energy, a leading generator in California, told the State Legislature that the company shut production units to drive up prices.

The energy agency has an obligation under federal law to ensure "just and reasonable" electricity prices. It has found that electricity prices in California have not met that standard, but recently it began imposing price controls.

Some analysts have said they suspected generating companies kept generating units off-line, limiting competition and forcing California's grid operator to pay stratospheric prices to avoid blackouts. The accusations were supported by circumstantial evidence that showed supplies of electricity declining this year when compared with the year before.

Generating companies have said they are doing all they can to run aging power plants at full capacity to meet demand. They say they shut units only for routine maintenance.

The energy agency conducted inspections and telephone inquiries at several California generating plants. It found the cutoffs were justified.

Curt Hébert Jr., the energy agency's chairman, wrote a letter to the accounting office, saying he agreed with its "basic findings." But he said his agency's study was never intended to examine all accusations of withholding but only to explore whether price manipulation had occurred in the few cases that were examined.