(Bloomberg) -- Chipotle Mexican Grill Inc., strugglingto rebound from multiple outbreaks of foodborne illness, reported abigger plunge in fourth-quarter sales than the restaurant chainexpected and said it got served with a subpoena in a federalcriminal investigation.

Same-store sales — a closely watched benchmark— tumbled 14.6% in the period, the Denver-based company saidin a regulatory filing on Wednesday. The decline, Chipotle’s firstquarterly drop as a public company, was previously projected to beas much as 11%. The grand jury subpoena, meanwhile, stemmedfrom a probe into a norovirus outbreak by the U.S. Attorney’sOffice for the central district of California, which is workingwith the Food and Drug Administration’s Office of CriminalInvestigations.

Chipotle’s reputation has been hammered in recent months by anoutbreak of E. coli that afflicted at least 53 people in ninestates. That was followed by a norovirus contagion at a Bostonlocation that sickened more than 140 college students. A new spateof illnesses in three additional states was announced by theCenters for Disease Control and Prevention late last month.

The shares fell 5% to close at $426.67 in New York , reachingtheir lowest point in more than two years. The stock declined 30%in 2015.

In the wake of the Boston outbreak — and the national newscoverage that followed — same-store sales fell 34%, thecompany said on Wednesday. After the CDC report on the additionalE. coli cases in late December, the sales dropped 37%. They weredown 30% for the full month ofDecember, Chipotle said.

Earnings estimate

The company now estimates that fourth-quarter earnings willrange from $1.70 to $1.90 a share. Analysts predicted $2.49 beforethe announcement, according to data compiled byBloomberg.Chipotle also expects to record $14 million to $16million in expenses during the quarter because of the crisis. Thecompany has had to replace food in restaurants, conduct lab testsand boost marketing to cope with the situation.

Chipotle also is making more share buybacks as it tries toplacate investors. The board agreed to repurchase $300 million instock, in addition to $300 million announced on Dec. 4.

California case

The subpoena pertains to a norovirus case ata Chipotle in Simi Valley, California, in August. Thatoutbreak sickened at least 234 customers and employees, though itdidn’t generate much national publicity before the E. coliincidents.

The illnesses in California were caused by a employee who hadthe highly contagious virus but wasn’t sent home from work. SteveElls, Chipotle’s co-chief executive officer, recently toldBloomberg News that the norovirus outbreaks in California andBoston involved breaches of protocol that were established in 2008after the virus sickened 500 people near Kent State University inOhio. Norovirus is the leading cause of illness from contaminatedfoods, affecting as many as 21 million people in the U.S. everyyear. Its symptoms, including diarrhea, vomiting and stomach pain,usually last a couple of days.

Chris Arnold, a spokesman for Chipotle, declined to commenton the investigation, as did the U.S. attorney’s office thatissued the subpoena. The FDA said it couldn’t provide more detailson the investigation.

“When foodborne illness outbreaks occur, the FDA works closelywith other federal and state agencies and other health officials toidentify the source, ensure the companies are removing affectedfoods from the marketplace, and communicate with the public,” theagency said in a statement. “No system is foolproof, but the FDAworks to prevent as many foodborne illness outbreaks aspossible.”

‘Range of documents’

In the filing, Chipotle said it’s been asked toproduce a “broad range of documents” as part of the subpoena inCalifornia. “It is not possible at this time to determine whetherwe will incur, or to reasonably estimate the amount of, any fines,penalties or further liabilities in connection with theinvestigation pursuant to which the subpoena was issued,” thecompany said.

The grand jury probe isn’t the first legal tanglefor Chipotle. It’s already involved in a U.S. governmentimmigration investigation that started in 2010 and caused thecompany to fire hundreds of workers. In 2012, prosecutors beganprobing possible criminal securities-law violations and subpoenaedthe company regarding work authorizations.

In the latest probe, it’s difficult to determine what potentialcrime is being investigated, according to Bill Marler, a Seattlelawyer who has worked on food-safety cases for more than 20 yearsand is representing about a dozen customers who got sick aftereating at the Simi Valley restaurant. He also was unsure whyfederal authorities have jurisdiction over a case that appears toinvolve a single restaurant in California. There have only been ahandful of criminal investigations into these kinds of food-safetyproblems, and they typically involve products shipped across statelines, he said.

“I don’t see what the federal jurisdiction is,” he said. “It’snot like they were shipping burritos to Reno.”

--With assistance from Leslie Patton and Andrew Harris.

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