NEW YORK — Chipotle temporarily closed a restaurant in a Washington, D.C., suburb after reports that customers had become ill, news that sent its shares down more than 6 percent as skittish investors worried about the chain’s past food scares.

The company says it shut down a location in Sterling, Va., on Monday after learning of a “small number” of reported illnesses with symptoms consistent with norovirus. It said it planned to reopen the restaurant after a “complete sanitization,” but did not provide a specific time.

Chipotle Mexican Grill says it is working with health authorities to understand the cause of the illnesses. The company notes that norovirus, which can cause nausea, stomach pain and diarrhea, does not come from its food supply and said it is safe to eat at its restaurants.

Its shares nonetheless fell 6.4 percent Tuesday to $366.86, after Business Insider reported the closure. Given its history, Chipotle has previously said that 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.

The company 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 spent more on 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.