A Chipotle Mexican Grill is seen in Los Angeles, California, U.S. on April 25, 2016. REUTERS/Lucy Nicholson/File Photo

(Reuters) - Chipotle Mexican Grill Inc CMG.N said U.S. federal prosecutors served a subpoena, seeking information related to a suspected norovirus outbreak at a restaurant in Sterling, Virginia that it briefly closed last week.

The burrito chain’s shares were down 1.8 percent at $342.50 in afternoon trading on Wednesday.

The latest subpoena is part of an ongoing criminal investigation being conducted by the U.S. Attorney’s office for the Central District of California into a spate of E. Coli, salmonella and norovirus outbreaks at the company’s outlets in late 2015 that sickened more than 100 people across states.

The July 19 subpoena is seeking information related to the incidents of illnesses associated with the Virginia restaurant, the company said in a filing.

More than 100 diners had reported norovirus like symptoms on iwaspoisoned.com, a crowd-sourced website, after eating at the restaurant.

Chipotle, which reported a better-than-expected profit on Tuesday, said it had identified a sick employee as the cause for the outbreak.

The company said it was retraining kitchen crews on food safety and enforcing a zero-tolerance policy for employees refusing to abide by new rules after the incident..

Up to Tuesday’s close, the company’s shares were down 6.5 percent since the Sterling incident was reported last Wednesday.