In this Dec. 15, 2015, file photo, a Chipotle Mexican Grill employee prepares food, in Seattle. After an E. coli outbreak that sickened more than 50 people, Chipotle is changing its cooking methods to prevent the nightmare situation from happening again. (AP Photo/Stephen Brashear, File)

Criminal investigation underway at Chipotle

Chipotle says it has been served with a federal grand jury subpoena as part of a criminal investigation tied to a norovirus outbreak this su…