Three people have died, and three others are in hospital after drinking milkshakes contaminated with listeria bacteria in the US state of Washington, health officials say.

Investigators found the bacteria in the ice cream machines at Frugals, a popular burger and milkshakes restaurant in Tacoma.

The restaurant in a statement said it was “deeply saddened” by the news.

Listeria can affect people up to 70 days after consuming contaminated food.

Health officials said the ice cream machines at the Tacoma restaurant were not cleaned correctly resulting in a foodborne listeriosis outbreak.

Six people were sent to the hospital between 27 February and 22 July, the Washington State Department of Health said. Genetic fingerprinting of bacteria in the milkshakes shows it was the same strain of Listeria.

Two of the three who survived said they had a milkshake from the same Frugals restaurant before feeling symptomatic.

In a statement, Frugals said: “We are heartbroken and deeply regret any harm our actions could have caused.”

“We are committed to making any changes deemed necessary to maintain our high standard of operations and prevent this from happening again.”

The Centers for Disease Control and Prevention estimates about 1,600 Americans get the infection every year, and some 260 die.

Symptoms of Listeria infection include fever, muscle aches, confusion and seizures. For those who are pregnant, symptoms can also include pregnancy loss or premature birth.

However, most people who eat Listeria-contaminated foods will not get sick, according to health officials, except for older populations, those who are pregnant, and the immunocompromised.

No other Frugals restaurants are believed to have the bacteria, officials said.

At least one lawsuit has been filed in the case, reported by local media, on behalf of a woman who claimed her husband suffered neck and arm pain until his death after drinking a Frugals milkshake.

Food safety lawyer Bill Marler representing the family of Charles Roberson said: “This is a really sad situation. You know, a husband and wife married for 45 years.”

“You can’t undo a death, but what you can do is make everything transparent so we can learn from this unfortunate incident,” he said.