Doug Schultz, spokesman for the Minnesota Department of Health, said the department confirmed 27 cases and received reports of four more likely cases.
Both Burger King sites voluntarily decided to close on Thursday.
Most of the cases were identified in September, he said, but the victims may have been exposed to Salmonella before that date. Two additional cases came to light this week, causing closures.
"It's possible that they were sick a couple of weeks before that," Schultz said. "It takes a while before people have symptoms, and then they're sick enough to go to the doctor, and then we identify."
The symptoms of salmonella, a bacterial disease that infects the intestinal tract, can include nausea and vomiting, abdominal cramps, diarrhea, fever, chills, headache and blood in the stool, according to the Mayo Clinic.
Once the Department of Health identified the outbreak in September they launched "strict" interventions, Schultz said. The Burger Kings were cleaned and employees with symptoms stopped working until they were symptom-free for 72 hours, but precautions did not stop the outbreak.
"Most of the time that does the trick, 98 percent of the time we do not see any more transmission," Schultz said. "In this case we had two additional illnesses this week."
Schultz said that the MDH believes that the outbreak is due to employee illness, rather than a specific element of contaminated food. Before restaurants can reopen, each employee must undergo the Salmonella test twice.
"Clearly we have evidence that the food worker's illness is part of the problem," Schultz said.
The earliest the Burger Kings could reopen is next week, according to Schultz. The restaurants must also be completely cleaned. The restaurants are located at 1000 Paul Bunyan Drive SW and 2575 Hannah Ave. NW.
Attempts to reach Burger King restaurants for comment were unsuccessful.