Chipotle Mexican Grill has been given a federal subpoena last month as part of criminal investigation linked to an outbreak at a restaurant of the company. The legal document asks Chipotle to provide documents of its Simi Valley restaurant where outbreak was reported a few months ago.

The US Attorney’s Office for the Central District of California and the Food and Drug Administration’s Office of Criminal Investigations have been investigating the case. The office has demanded documents from his office regarding the Denver based Chipotle’s case, said Doug Beach from Ventura County’s Environmental Health Division.

Beach also said that Chipotle has been cooperating with the investigation, which already revealed issues such as unclean equipment in restaurants and employees without the essential food handling permits. “Chipotle started getting complaints about illnesses on Tuesday, Aug. 18, and shut down its restaurant the following Friday. Yet the company did not alert the county of the matter until Saturday – after it had already reopened the restaurant”, Beach explained.

Chipotle has been facing outbreaks since last few months. The norovirus outbreak also affected the company’s shares, which are dwindling continuously, and its sales. The cases liked to the outbreak received far more national media attention.

In a release on Wednesday, Chipotle revealed that December sales have fallen about 30% at some established locations. The full fourth quarter sales were also down by more than 14% at these established locations, the company said. It has been the first decline for the company since it went public more than nine years ago. The fall also forced Chipotle to cut its profit outlook for the period.

Chipotle said in a statement in December that it could no longer predict sales trends given the food scares.