Dynegy

(DYN)

was heading south again Wednesday after news broke that the

SEC

plans a formal investigation into a gas-supply deal that has been the source of investor nervousness for months.

Houston-based Dynegy said it would "continue to cooperate fully" with the Securities and Exchange Commission, which had previously launched an informal probe of the company's five-year natural gas deal with ABG Gas Supply LLC.

Questions have surrounded the agreement, known as "project Alpha," which reportedly saved Dynegy some $80 million in taxes. Several shareholder suits were filed when the informal investigation was revealed and Standard & Poor's downgraded the company's debt.

Dynegy's shares were recently down $2.26, or 18%, to $10.