Chipotle Mexican Grill Inc. plunged on Thursday after reporting it received a subpoena related to an outbreak of customer illnesses last year -- bringing a sensitive issue back into the spotlight for the burrito chain.

The company said in a filing that the subpoena, received April 18, requested information about an outbreak last summer linked to a restaurant in Powell, Ohio. The company temporarily closed the restaurant for cleaning after more than 600 customers reported getting sick with symptoms including nausea, diarrhea and cramping.

Chipotle shares fell as much as 7.2 per cent -- the most in four months. The company has seen its stock soar since Chief Executive Officer Brian Niccol took the reins as reports of food-related illness outbreaks have fallen compared with previous years.

“We have fully cooperated and intend to continue to fully cooperate in the investigation,” Chipotle said in the filing.