Things are evidently not improving for Chipotle Mexican Grill, the fast-casual chain announced on Wednesday that it was handed a grand jury subpoena as part of a federal investigation into an outbreak of norovirus at a location in California.

In the same company memo, the restaurant said its comparable restaurant sales were down 30 percent in December as a result of national media attention over a string of E. coli and norovirus outbreaks at its stores nationwide.

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The news came in the form of an SEC filing on Wednesday, and provided a glance at the latest in a series of public relations nightmares for the company.

"The subpoena requires us to produce a broad range of documents related to a Chipotle restaurant in Simi Valley, California, that experienced an isolated norovirus incident during August 2015," the filing reads. "We intend to fully cooperate in the investigation."

The case for which the subpoena was issued is a criminal one, being conducted by the U.S. attorney's office for the Central District of California and the U.S. Food and Drug Administration's Office of Criminal Investigations. While the charges in the case are yet to be laid out, the criminal nature of the investigation could mean the company would be on the hook for deliberately contaminating food, depending on the impending grand jury decision, Business Insider reported.

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A spokesperson for Chipotle, Chris Arnold, did not immediately return ATTN:'s request for comment. The U.S. attorney's office in California declined to comment on the case.

Chipotle suffered from a string of E. coli outbreaks beginning in October in restaurants in the Pacific Northwest, and later in other parts of the country.

In addition to the E. coli cases, a separate norovirus outbreak in Boston sickened more than 100 people who ate at the restaurant. According to an internal report obtained by Food Safety News, the most recent California norovirus case was much larger, sickening about 234 people.

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The contamination cases have prompted the company to revise many of its cooking methods, but have also taken a toll on the chain's finances. Sales declined in the fourth quarter in Wednesday's SEC filing bigger than previously predicted. Chipotle's stock has dropped about 40 percent in recent months.