Seekonk, MA – Panera Bread Co. shuttered a restaurant in Massachusetts after employees and customers there were possibly exposed to carbon monoxide.

The franchisee who operates the restaurant in the town of Seekonk is working with the health department to investigate, Panera said in a statement. The cafe will remain closed “until we are absolutely certain it is safe,” the company said.

“Earlier today, several guests and associates at the Panera bakery-cafe in Seekonk exhibited symptoms consistent with possible carbon monoxide exposure,” the company said. “They were immediately taken to the hospital to be evaluated.”

Panera shares declined as much as 1.8 percent when the news of sickened customers first surfaced. The stock began to rebound after the cases were tied to carbon monoxide, rather than food. As of 2:48 p.m. in New York, the shares were trading at $220.25, down 0.6 percent.

Restaurant investors have been on edge about foodborne illnesses since Chipotle Mexican Grill Inc. suffered E. coli and norovirus outbreaks last year.

Panera said that 13 patients were treated by rescue personnel at the scene, and five were taken to the hospital for further treatment as a precaution.