Officials with Chipotle Mexican Grill say they will “fully cooperate” after receiving a subpoena from a federal grand jury seeking documents related to a norovirus outbreak at a restaurant in Simi Valley, Calif. Chipotle Mexican Grill signThe subpoena, issued in December 2015, came out of a grand jury from the U.S. District Court for the Central District of California, according to a document Chipotle filed with the Securities and Exchange Commission on Jan. 6. Immediately following the SEC filing, Chipotle’s stock took another tumble, falling nearly 3 percent to $435.72 in early trading, Reuters reported. In July, before a series of six foodborne outbreaks linked to the chain began, Chipotle shares were selling for $750. Chipotle’s stock value dropped by $6 billion this fall in the midst of the string of outbreaks that sickened more than 500 people across 10 states. Two outbreaks involved norovirus, three of them were caused by E. coli and the other was caused by Salmonella, according to the Centers for Disease Control and Prevention. Chipotle has not updated outbreak information on its website since Dec. 21. Investigators from the U.S. Attorney’s office for California’s central district are working in conjunction with the Food and Drug Administration’s Office of Criminal Investigations on the Simi Valley case. The norovirus outbreak in August in Simi Valley sickened at least 234 people, according to public health records. It was the second of six foodborne illness outbreaks traced to Chipotle restaurants from July to December 2015, but Chipotle characterized it as “isolated” in its SEC filing. “The subpoena requires us to produce a broad range of documents related to a  Chipotle restaurant in Simi Valley, Calif., that experienced an isolated norovirus incident during August 2015,” according to the SEC filing. “It is not possible at this time to determine whether we will incur, or to reasonably estimate the amount of, any fines, penalties or further liabilities in connection with the investigation pursuant to which the subpoena was issued.” The fast food company’s sales are falling in the wake of “national media attention,” according to its SEC filing. On Dec. 4 Chipotle reported to the SEC that comparable restaurant sales were down 16 percent. A norovirus outbreak in Massachusetts the second week of December sent sales down an average of 34 percent, the company reported to the SEC. The sixth Chipotle-related outbreak — reported by the CDC in late December — further decreased business. “Following (the CDC) announcement and related national media attention, our comparable restaurant sales trended down to -37 percent. For the full month of December, comparable restaurant sales were -30 percent. Future sales trends may be significantly influenced by further developments,” according to the SEC filing. Chipotle officials reported non-recurring fourth-quarter expenses for 2015 related to the outbreaks would be in the range of $14 million to $16 million. “The estimate of non-recurring expenses includes costs to replace food in select restaurants, lab analysis of food samples and environmental swabs, increased marketing expenses, retaining expert advisory services related to epidemiology and food safety and preliminary estimates for legal claims and related expenses,” according to the SEC filing.