Chipotle Mexican Grill Inc., the Denver-based chain staggering from a string of food-borne illness outbreaks at its restaurants, is under federal criminal investigation for a norovirus outbreak at one of its California locations.

The August outbreak occurred at a restaurant in Simi Valley and sickened 189 customers and 18 employees, according to a local health department worker who spoke with NPR. After the outbreak, local authorities reported finding meat held at improper temperatures and dirty conditions in the restaurant’s kitchen. A subpoena from a federal grand jury, disclosed Wednesday in a filing with the Securities and Exchange Commission, requires Chipotle to produce a range of documents related to the outbreak.

The US Attorney's Office for the Central District of California and the Food and Drug Administration are jointly conducting the investigation. In the SEC filing, Chipotle said it intends to fully cooperate, though it’s unclear at this point what criminal violations or charges might be pursued.

News of the criminal probe follows a series of outbreaks for the chain. In the past several months, nearly 60 customers from 12 states became ill from Shiga toxin-producing Escherichia coli O26 (STEC 026) linked to contaminated food at Chipotle. STEC 026 causes severe diarrhea, abdominal pain, and in some severe cases, kidney failure. So far, the source of the contamination is still unknown.

In December, another norovirus outbreak was reported at a Chipotle in Boston. More than 150 people were sickened in that outbreak. Norovirus, a common and easily transmitted germ, causes diarrhea, cramps, and vomiting.

News of the outbreaks has hurt sales. On Wednesday, Chipotle revealed that its sales fell by 14.6 percent in the last quarter of 2015.

Chipotle has apologized for the outbreaks and made plans to revamp food safety at all of its locations nationwide.