The Centers for Disease Control and Prevention said on Monday that five more people became sick after eating at two more Chipotle Mexican Grill restaurants, dealing a further blow to the price of the company’s shares.

The C.D.C. said that five people who had eaten in two Chipotle restaurants in Kansas and Oklahoma grew ill after eating something contaminated with E. coli STEC O26. The agency has not yet determined which food is responsible for the outbreak.

“One of the challenges here has been that we have been able to identify the restaurants where people ate, but because of the way Chipotle does its record-keeping, we have been unable to figure out what food is in common across all those restaurants,” said Dr. Ian Williams, chief of the outbreak response and prevention branch of the C.D.C.

The same bacteria has sickened 53 people in eight other states, nearly all of whom said they had eaten at a Chipotle. But Dr. Williams said the DNA of the bacteria appeared different in the latest outbreak than in the previous one.