Restaurant chain Chipotle Mexican Grill has temporarily closed an outlet in Massachusetts following concerns over the outbreak of a food-borne illness as four employees at the eating joint have reported ill. The latest setback has come even as the food chain was already striving hard to recover from a series of related illness outbreaks which impacted its business few months back.

The restaurant has been closed voluntarily and so far there are no known customers who have suffered any illness, said the Massachusetts health department.

Chipotle Mexican Grill spokesman Chris Arnold said in an e-mail that the restaurant where the employees fell ill has been located in the town of Billerica outside Boston and the facility has been closed for a full cleaning.

The restaurant chain has been struggling under a food-safety crisis since November after the Centers for Disease Control and Prevention started investigating the chain’s restaurants for a possible E. coli outbreak. In December, scores of college students fell ill in Boston following a norovirus outbreak, which affected Chipotle’s sales. These developments brought under focus the previous food-borne illnesses cases reported by customers after having food at the chain’s outlets.

Arnold said on Tuesday that it was not clear yet whether the sick employees were affected by norovirus as no reports of customers falling ill had come to the fore. The spokesman said employees who had taken ill will be tested and kept out of the restaurant till they recover fully.

The incident has come as an economic setback for Chipotle as the company’s shares fell 4.7% to $500.09 — the stock has fallen 20% over the past one year — in late trading after the news of the shutdown of the restaurant was reported by the local media.

Chipotle has been working hard to improve its reputation after the setback due to two E.Coli outbreaks that had sickened about 50 people in 14 states and two separate norovirus outbreaks in Massachusetts and California earlier.