Two Burger King restaurants in Bemidji have been forced to shut their doors following a salmonella outbreak.

Minnesota Health Department tells the Star Tribune that since September, they’ve confirmed 27 cases of salmonella linked to the two fast-food restaurants.

Officials say there are four other possible cases. Both of the Burger King locations closed voluntarily Thursday to undergo a thorough decontamination process.

Liz Sawyer of the Star Tribune writes that 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.