Chipotle has been served with a federal grand jury subpoena requesting information related to a norovirus outbreak at its restaurant in Sterling, Virginia.
The subpoena is part of a larger federal criminal investigation into food-related illnesses at Chipotle restaurants dating back to 2015.
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Chipotle said in a filing Wednesday that it intends to fully cooperate with the investigation.
"It is not possible at this time to determine whether we will incur, or to reasonably estimate the amount of, any fines or penalties in connection with the investigation pursuant to which the subpoena was issued," the company said.
The investigation is being conducted by the US attorney's office for the Central District of California and the US Food and Drug Administration's Office of Criminal Investigations.
A look at Chipotle restaurants through the years:
The subpoena means that prosecutors are gathering evidence to present to a grand jury, which will decide whether to make an indictment. The charges have not been specified.
Criminal investigations into outbreaks of foodborne illness are uncommon.
The norovirus outbreak linked to the Chipotle restaurant in Virginia sickened at least 100 customers, according to iwaspoisoned.com, a website that allows people to self-report suspected food poisoning.
Norovirus is highly contagious and causes symptoms like stomachaches, nausea, diarrhea, and vomiting. It's the most common cause of foodborne illnesses in the US, with more than 21 million cases annually.
In addition to the norovirus outbreak in Sterling, Virginia, Chipotle has been identified as the source oftwo E. coli outbreaks, a norovirus outbreak in Boston in December 2015, and a norovirus outbreak linked to a Simi Valley, California, Chipotle restaurant in August 2015.
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