NEW YORK (AP) — Chipotle has been served with a federal subpoena as part of a criminal investigation tied to a norovirus outbreak at one of its restaurants in California.

The investigation is being conducted by the U.S. Attorney’s Office for the Central District of California in conjunction with the Food and Drug Administration’s Office of Criminal Investigations, the company said in a filing with the Securities and Exchange Commission Wednesday.

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The subpoena, received last month, requires the company to produce a broad range of documents tied to a restaurant in Simi Valley, California, that was the source of a norovirus outbreak this past August, it said.

A Chipotle spokesman, Chris Arnold, said in an email the company does not discuss pending litigation, but that it intends to cooperate fully with the investigation.

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Chipotle Mexican Grill Inc. has been reeling since an E. coli outbreak linked to its restaurants in late October and November, which was followed by a separate norovirus outbreak at a restaurant in Boston in December. The cases have received far more national media attention than the norovirus outbreak in California.

Chipotle said sales plunged 30 percent in December. The company expects sales to fall 14.6 percent at established locations for the full fourth quarter, marking the first decline since the company went public in 2006.

To rehabilitate its image, the company has taken out full-page ads apologizing to customers in dozens of newspapers around the country. It has also vowed changes to step up food safety at its restaurants.

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