(Bloomberg) -- Chipotle Mexican Grill Inc., struggling to rebound from multiple outbreaks of foodborne illness, reported a bigger plunge in fourth-quarter sales than the restaurant chain expected and said it got served with a subpoena in a federal criminal investigation.

Same-store sales — a closely watched benchmark — tumbled 14.6% in the period, the Denver-based company said in a regulatory filing on Wednesday. The decline, Chipotle’s first quarterly drop as a public company, was previously projected to be as much as 11%. The grand jury subpoena, meanwhile, stemmed from a probe into a norovirus outbreak by the U.S. Attorney’s Office for the central district of California, which is working with the Food and Drug Administration’s Office of Criminal Investigations.

Chipotle’s reputation has been hammered in recent months by an outbreak of E. coli that afflicted at least 53 people in nine states. That was followed by a norovirus contagion at a Boston location that sickened more than 140 college students. A new spate of illnesses in three additional states was announced by the Centers for Disease Control and Prevention late last month.