Chipotle Mexican Grill Inc. voluntarily shut down a restaurant in Billerica, Mass. after three employees became sick, the chain said on Tuesday.

“After learning that four of our employees were not feeling well, our restaurant in Billerica, Mass. was closed for a full sanitization. No customers illnesses are connected to this restaurant,” said Chipotle spokesman Chris Arnold. “Any employees who reported feeling ill will be held out of the restaurant until they fully recover.”

A local news report that broke the news said norovirus was a concern, but Massachusetts Health and Human Services spokesman Scott Zoback said the source of the illnesses had not been confirmed.

“It’s a voluntary shutdown to help with the investigation, which is being led by the Billerica health department,” said Zoback.

This news comes just as the Denver-based Chipotle launches a new campaign to bring customers back to their restaurants following foodborne illness outbreaks this winter.

Last month the Centers for Disease Control and Prevention announced that the two E. coli outbreaks that sickened 60 consumers appeared to be over.

Two Chipotle restaurants, in Boston and California, also had norovirus outbreaks and one restaurant in Minneapolis had a salmonella outbreak.

The chain's food-safety troubles led to a 14.6 decline in same-store sales for the fourth quarter ending Dec. 31.

In February Chipotle shut down its 1,971 stores for lunch for a companywide meeting to talk to employees about its new food-safety initiatives.

Chipotle has set the goal of being a leader in food safety with more fresh food testing, new food safety practices and a commitment for $10 million to train farms on food safety standards.

—Reporting contributed by Lisa Jennings

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