Chipotle was served with a federal grand jury subpoena as part of a criminal investigation tied to a dangerous norovirus outbreak this summer at one of its restaurants in California, the company announced Wednesday. 
 
 The subpoena, received last month, requires the company to produce a broad range of documents tied to the restaurant in Simi Valley. 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, the company said. 
 
 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. 
 
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 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. 
 
 People can get norovirus from an infected person, contaminated food or water, or by touching contaminated surfaces, according to the Centers for Disease Control and Prevention. The virus is very contagious and can spread quickly in places such as daycare centers and cruise ships, the agency says. Each year, it causes 19 to 21 million illnesses. 
 
 The Centers for Disease Control and Prevention said the outbreak of E. coli tied to Chipotle has sickened at least 52 people in nine states. The first cases were reported at the end of October in Oregon and Washington. Chipotle Mexican Grill Inc. has more than 1,900 locations, primarily in the U.S. 
 
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 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. 
 
 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. 
 
 The Associated Press contributed to this report. ||||| 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. 
 
 This Sunday, Dec. 27, 2015, photo, shows a Chipotle restaurant at Union Station in Washington. Chipotle said Wednesday, Jan. 6, 2016, it has been served with a federal grand jury subpoena as part of a... (Associated Press) 
 
 The subpoena, received last month, requires Chipotle to produce a broad range of documents tied to a restaurant in Simi Valley, California, where a norovirus outbreak took place this past August, the company said in a regulatory filing Wednesday. 
 
 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, Chipotle said in a filing with the Securities and Exchange Commission. 
 
 Representatives from those offices were not immediately available for comment. 
 
 The disclosure comes as Chipotle Mexican Grill Inc. reels from an E. coli outbreak linked to its restaurants in late October and November. That was followed by a separate norovirus outbreak at a restaurant in Boston. The cases received far more national media attention than the norovirus outbreak in California, and the company's sales have plunged. 
 
 Sales fell 30 percent at established locations in December, Chipotle said Wednesday. 
 
 The Denver company said sales for the full fourth quarter were down 14.6 percent at established locations, marking the first decline since the company went public in 2006. It also cut its profit outlook for the period. 
 
 Last month, Chipotle said it could no longer reasonably predict sales trends given the food scares. It retracted its sales forecast for 2016. 
 
 In its regulatory filing Wednesday, the company said it could not determine or predict the amount of any "fines, penalties or further liabilities" it might face in connection with the federal investigation. 
 
 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. 
 
 The emergence of a criminal investigation after a norovirus outbreak is unusual, said Bill Marler, a food safety lawyer representing Chipotle customers who were sickened in Simi Valley. 
 
 Outbreaks at restaurants are typically caused by an infected employee. 
 
 Marler couldn't think of a reason for a federal investigation, other than employment violations. 
 
 To rehabilitate its image, Chipotle has taken out full-page ads apologizing to customers in dozens of newspapers around the country. It also vowed changes to step up food safety at its restaurants, in part by tweaking its cooking methods and increasing testing of meat and produce. 
 
 Co-CEO Steve Ells has said the company will likely never know what ingredient was to blame for the E. coli cases. 
 
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 Follow Candice Choi at www.twitter.com/candicechoi ||||| Federal authorities are conducting a criminal probe tied to an outbreak of foodborne disease at one of Chipotle Mexican Grill Inc.’s restaurants last year, the company said, raising the stakes in a series of such outbreaks that have battered Chipotle’s sales. 
 
 The company said in a securities filing Wednesday that it was served with a grand-jury subpoena in December seeking information about a norovirus outbreak at a Simi Valley, Calif., restaurant in August that sickened 234 people, including 17 employees. 
 
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– Sickened customers and employees, bad publicity, and tumbling stock prices were apparently just the beginning of Chipotle's woes. Now the restaurant chain has been slapped with a federal subpoena as part of a criminal investigation into a food poisoning outbreak in August, the AP reports. The subpoena received in December mandates that Chipotle produce "a broad range of documents" in regard to a norovirus outbreak at its restaurant in Simi Valley, Calif., which Bloomberg notes sickened 234 customers and workers. In an SEC filing, Chipotle says the investigation is being performed by the US Attorney's Office for the Central District of California and the FDA. A food safety lawyer says that a criminal probe following such an outbreak isn't typical, and that the only reason the feds might get involved is if there were employment infractions. This case isn't even the most publicized of Chipotle's recent health scares: A norovirus outbreak in Boston sickened more than 140 college students in late 2015, and an E. coli outbreak in nine states sickened more than 50, Bloomberg notes. The company's numbers are already hurting: Shares are down 37% over the past three months, per the Wall Street Journal, and Fox News reports that Chipotle predicts sales will fall 14.6% for 2015's fourth quarter—the first drop since the company went public in 2006. Chipotle says future sales could be "significantly influenced by further developments" and notes it will report sales for the quarter and full year on Feb. 2, per the Journal. A Chipotle rep tells the AP it will cooperate fully with the probe. (Here's how Chipotle is trying to fix its food issues.)