SAN FRANCISCO — Diamond Foods, Inc. said in a filing on Dec. 15 that it is the subject of a U.S. Securities and Exchange Commission investigation. The company did not say what the S.E.C. is investigating, but Diamond Foods is currently conducting an internal investigation related to accounting for certain crop payments for walnut growers. The company said it will fully cooperate with the S.E.C.

The S.E.C. investigation comes a few days after Diamond Foods said it would not meet the deadline to file its 10-Q form for its fiscal first quarter. The delay is due to the investigation of the company’s audit committee into the payment to the walnut growers.