Chipotle Mexican Grill said it received a follow-up subpoena relating to the norovirus outbreak at its Sterling, Virginia, restaurant earlier this month.
In a filing with the Securities and Exchange Commission, the company disclosed it received the subpoena on July 19, and plans to "fully cooperate in the investigation."
Several customers were sickened at the restaurant and testing last week confirmed that the illness was norovirus.
During an earnings conference call Tuesday, Chipotle executives disclosed that they attributed the outbreak to lax enforcement of the company's sick policy by store managers and a sick worker at the restaurant.
"We conducted a thorough investigation, and it revealed that our leadership there didn't strictly adhere to our company protocols," CEO Steve Ells said during the call.
Chipotle plans to embark on a training program to more stringently enforce the policies it has regarding the handling of food in its restaurants.
Ells said Tuesday that the company's procedures are "nonnegotiable and a condition of their employment."
This is the latest setback for the Mexican restaurant chain, which was just starting to show progress in winning back customers after a string of food safety illnesses, beginning in 2015, scared diners away.
In January 2016, Chipotle was served with a federal grand jury subpoena as part of a criminal investigation of a norovirus outbreak at a California restaurant. As part of that probe, the company was asked to supply documents and information regarding its food safety practices dating back to January 2013.
"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," the company said, in regard to its latest subpoena.
Chipotle shares were recently trading down 1 percent.