fell 26% last week after news broke that the

Securities and Exchange Commission

would launch a formal investigation into a gas-supply deal that has been the source of investor nervousness for months. The SEC is investigating electric-power trades carried out by Dynegy last fall. The trades, which were executed simultaneously for the same price, didn't yield any profit for Dynegy but propped up the company's trading volume at a time when the company was trying to muscle in to the industry's top spot following