NEW YORK (AP) — Chipotle's efforts to move past its food scares have been complicated by fresh reports of illnesses, which prompted it to temporarily close a restaurant this week.

The company said Tuesday that it closed the restaurant in Sterling, Virginia after it became aware of a "small number" of reported illnesses consistent with norovirus. The news sent its shares down more than 4 percent as skittish investors worried about the chain's past food scares.

Chipotle noted that norovirus, which can cause nausea and diarrhea, does not come from its food supply and said it is safe to eat at its restaurants. But the company has previously said that given its history, any food safety incidents could have an outsized negative impact on its sales, even if reports turn out to be erroneous or stem from factors outside its control.

Its shares fell $17.02 to close at $374.98, after Business Insider reported the closure.

The company said it planned to reopen the restaurant, which is in a suburb of Washington, D.C., on Tuesday after a "complete sanitization," but did not provide a specific time.

Chipotle has been working to bounce back from food scares that included an E. coli outbreak in the fall of 2015 and a norovirus case in Boston later that year. It subsequently said that it made tweaks to cooking methods and added training for employees to tighten its safety measures.

The Denver-based company also gave away coupons for free burritos and stepped up marketing to win back customers. Sales had been showing improvement against a low bar of comparison. For the first three months of this year, sales were up 18 percent at established locations. That followed a 20 percent decline for all of 2016.

Despite its efforts, Chipotle has said it may still be at higher risk for food-borne illnesses than others because of its greater use of fresh produce and meats.

David Goodfriend, director of the Loudoun County Health Department in Virginia, said his office was contacted by Chipotle on Monday and told the company had voluntarily closed the location for a cleaning. Goodfriend said the health department also had been contacted by a couple doctor's offices and residents who reported symptoms.

While he isn't certain that the restaurant was the cause of the illnesses, Goodfriend said people were associating their symptoms with the chain.

Norovirus is the leading cause of illness and outbreaks from contaminated food in the United States, according to the Centers for Disease Control and Prevention. Infected employees are frequently the source of the outbreaks, the CDC says, often by touching foods such as raw fruits and vegetables with their bare hands before serving them.

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