NEW YORK – Chipotle said it has been subpoenaed as part of a federal criminal investigation and that sales plummeted 30 percent in December after a series of food scares at its restaurants.

The company said in a regulatory filing it was asked to produce a broad range of documents tied to a norovirus outbreak this summer at its restaurant in Simi Valley, California, but declined to provide further details.

It said the investigation does not involve a more recent E. coli outbreak tied to its restaurants or a separate norovirus outbreak in Boston.

The investigation is being conducted by the U.S. Attorney’s Office for the Central District of California with the Food and Drug Administration’s Office of Criminal Investigations, according to a filing with the Securities and Exchange Commission Wednesday.

Representatives for the U.S. Attorney’s office and the FDA declined to comment.

The disclosure of the investigation comes as Chipotle Mexican Grill Inc. reels from E. coli outbreaks in late October and November, which were followed by the sickening of customers at a restaurant in Boston in early December. Those cases received far more national media attention than the California incident, and the company’s sales have since plunged.