A listeria outbreak that led to the deaths of three people has been linked to milkshakes sold by the burger chain Frugals at its restaurant in Tacoma, Wash., according to the state’s health department.

In a news release on Friday, officials said that outbreak had been caused by the food-borne listeria, a type of bacteria that can cause serious sickness or death in people 65 or older and miscarriages and premature births in pregnant women. At least three other people were hospitalized as a result of the outbreak from Feb. 27 to July 22.

The same strain of the bacteria was found in ice cream machines at the restaurant, about 10 miles south of downtown Tacoma, that had not been properly cleaned, the health department said. The restaurant stopped using the ice cream machines after they were tested on Aug. 8, but listeria can sicken people several days after consuming the bacteria, health officials said. None of the other Frugals restaurants in Washington or Montana are believed to have been affected, they added.

In a statement posted on Instagram over the weekend, Frugals said, “We are heartbroken and deeply regret any harm our actions could have caused.” The Tacoma Frugals has stopped selling milkshakes and has sent the milkshake equipment to be cleaned and retested, it said.

Frugals did not immediately respond to requests for additional comment on Monday evening.

Investigators said that all of the six people hospitalized, including those who died, were immunocompromised, and that genetic fingerprinting of the bacteria had showed the same food was likely responsible for making them sick.

Two of those who were sickened and survived told investigators they had drunk milkshakes from the restaurant.

Past listeria outbreaks caused by ice cream and milkshakes prompted the Tacoma-Pierce County Health Department to take samples from the restaurant on Aug. 8, state health officials said. Ten days later, they confirmed that all of the restaurant’s milkshake flavors had been contaminated with the same strain of listeria that caused the outbreak.

While most people who eat food contaminated with listeria do not develop serious illness, state health officials have advised anyone who is pregnant, age 65 and older or immunocompromised and drank a milkshake at the restaurant from May 29 to Aug. 7 to contact their health provider.

“The milkshake machines will be kept out of service until the Tacoma-Pierce County Health Department determines they are free of Listeria contamination and no longer pose a danger to the public,” the Washington State Department of Health said.