Health officials say additional salmonella cases could emerge as the investigation continues into a national outbreak.
Preliminary tests have linked about 30 cases of salmonella poisoning in Minnesota to contaminated peanut butter.
Almost 400 people in 42 states have become sick.
Officials say they are waiting for lab results before they can say for sure whether the patients were sickened by the same strain.
The salmonella is traced to two brands of peanut butter distributed nationally to nursing homes, colleges and other institutions. Ohio-based King Nut has voluntarily recalled the brands linked to the illness.
Doug Schultz, spokesman for the Minnesota Department of Health, said it is not uncommon for new cases to surface after an outbreak of food poisoning is reported.
"With salmonella, most people, the symptoms will resolve within five to seven days so you could get sick and have a mild case of salmonella and get over it and not think too much about it, and then people will realize 'hey, I ate a peanut butter sandwich at the nursing home or the college or the school and got sick,'" Shultz said.
Schultz said the bacteria is among the most common strains of salmonella.