NEW YORK (CNN/Money) - Stocks of energy-trading companies got clobbered Wednesday amid renewed questions about accounting and creditworthiness -- the latest fallout in an industry tarnished by the collapse of Enron Corp.

Shares of independent power producer Calpine (CPN: Research, Estimates) sank almost 5 percent a day after rating agency Moody's Investors Service further cut its "junk" ratings on the company's senior debt by three notches, citing concerns about the debt-heavy company's liquidity.

Williams (WMB: Research, Estimates) shares also tumbled about 5 percent after it confirmed it received a letter from the Securities and Exchange Commission, which is probing the accounting practices of 49 companies. But the energy trader said it has no unresolved issues with the agency.

And Dynegy (DYN: Research, Estimates) shares sank 7 percent, following a report that said the company used complex accounting moves involving a gas partnership to help cut its tax bill and raise its reported cash flow for the year 2001.

Through a system dubbed "Project Alpha," Dynegy, whose businesses include energy marketing, trading and power generation, was able to increase its reported cash flow by $130 million instead of reducing it by $164 million, according to the Wall Street Journal. The Journal credited the cash-flow analysis to an unnamed fund manager who is short Dynegy stock. Short-sellers profit when a stock they bet against declines.

Project Alpha also enabled Dynegy to reduce its 2001 income-tax rate to 29 percent from 2000's rate of 34 percent, which the company says saved it $80 million on a one-time basis, the Journal said.

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In a response provided to CNN/Money's sister network CNN, Dynegy (DYN: Research, Estimates) spokesman John Sousa said Project Alpha was an economically sound transaction with a natural-gas supply transaction at its heart, executed in the normal course of business and accounted for appropriately. He noted that no related parties were involved in the transactions and said it was disclosed in 2001 financial statements.

He called Wednesday's stock-market reaction "a short-term overreaction."

The arrangement helped Dynegy with a problem resulting from an industry requirement to mark on their financial statements estimates of the current market value of energy-trading contracts, the article said.

Some analysts think those estimates were too generous for Dynegy and other industry participants in 2000, the article said. Nevertheless, those estimates boost the bottom line because they are added directly to net income as unrealized gains, it said. If the estimated value of the contracts doesn't materialize as cash flowing into the company, however, it must be deducted from cash flow.

The report said disclosure of Project Alpha in the company's 2001 annual report, filed March 13, excluded the project's name and was limited to mention of involved subsidiary DMT Supply LP, lines in a financial table labeled "operating margin" and "other expenses," and an item called "book-tax basis differences" in a section addressing tax payments.  

-- Reuters contributed to this report.