Two Burger Kings in Bemidji, Minn., have temporarily shut their doors following a salmonella outbreak that infected more than two dozen customers, health officials say.

Since September, the Minnesota Department of Health has identified 27 confirmed cases and another four probable cases of salmonella with links to the two fast-food restaurants. Both sites voluntarily closed to the public on Thursday to begin the decontamination process.

“Some of the extreme measures we’re taking are that all of their employees need to test negative for salmonella [twice], not sooner than 24 hours apart,” said Doug Schultz, a spokesman for the state Department of Health. “Once that’s done, we can do additional cleaning and they can open again.”

Once most cases were reported this fall, the Health Department imposed strict interventions that rigorously cleaned the restaurants and barred employees with symptoms from working for 72 hours.

Often that will be enough to stop further transmission, Schultz said. Yet two new cases were identified this week that appear to match outbreak strain — ultimately prompting the closures.

Salmonella bacteria can cause diarrhea, vomiting, stomach pain and fever. Young children, older adults and people with weakened immune systems are the most likely to contract severe infections.

It remains unclear whether the outbreak began with an infected food worker or a specific contaminated food item, Schultz said.

“We don’t know which came first. There was no clear source identified,” he said. “If they’re working while they’re ill — as hard as you try to wash your hands — it can spread.”

It is fairly unusual for salmonella to originate in fast-food restaurants, Schultz said. The bacteria tend to appear more often in ready-to-eat items that contain lettuce and other produce sold at supermarkets.

No other restaurants appear to be affected by the outbreak and health officials say there is no evidence to indicate there’s an ongoing issue with a particular type of produce. The Burger King locations may reopen by week’s end.