Last Updated Dec 21, 2015 4:01 PM EST
U.S. health officials are investigating Chipotle Mexican Grill (CMG) restaurants for a possible connection to an outbreak of a different strain of E. coli.
Five people have reported becoming ill in Kansas, North Dakota and Oklahoma, the U.S. Centers for Disease Control and Prevention said Monday in a statement. All said they ate at a Chipotle in the week before becoming ill.
The development comes less than two weeks after Steve Ells, Chipotle's founder and co-CEO, apologized for the E. coli outbreak, and pledged to tighten food safety procedures to ensure such incidents do not happen again.
A total of 53 people in nine states have taken ill in the outbreaks, which hit a peak in October, according to the CDC.
Separately, 141 students at Boston College were recently sickened by norovirus after eating at a Chipotle not far from campus.
Shares of the restaurant chain fell to a 52-week low of $508.10 following the CDC announcement on Monday. Its shares were down $19.07, or 3.5 percent, at $522.01 at the close.