NEW YORK ( TheStreet) -- Shares of Diamond Foods plummeted Thursday on news the snack company is being investigated by the Securities and Exchanges Commission.

The SEC investigation is in addition to Diamond Food's own internal audit regarding some crop payments to walnut growers. In a filing to the SEC, Diamond said it would cooperate with the SEC investigation. The San Francisco-based snack company said it received a formal order of investigation from the SEC on Wednesday.

"The SEC has informed Diamond that its investigation should not be construed as an indication by the SEC that any violations of law have occurred," the filing stated.

Diamond Foods' shares fell more than 20% on Dec. 12 when the company said its internal investigation would not be completed until February and its 10-Q would be delayed.

Diamond Foods' problems began when a former director, Joseph Silveira , died in November, possibly by suicide, amid accounting issues. The company also said in November that it would close the Pringles acquisition from Procter & Gamble later than December because its audit committee received some questions about accounting for the walnut grower crop payments. The Pringles deal is slated to close in the first half of fiscal year 2012.

Shares fell 9.5% to $26.70 on Thursday.

-- Written by Alexandra Zendrian

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