Five people have now died, and a further 197 taken ill, after a deadly E. coli outbreak in the US linked to tainted lettuce from Arizona.

The incident has been ongoing since March 13, with 35 states now affected. Two of the deaths were in Minnesota and one each in Arkansas and New York since Mid-May. Earlier in May, the first death was reported in California.

According to the Centers for Disease Control and Prevention (CDC) in an update on Friday, almost half of the 197 infected people have been hospitalized.

The issue appears to stem back to romaine lettuce grown in Yuma, Arizona. However, a particular farm or distributor has not yet has been found that started the outbreak. In a blog post, Scott Gottlieb, commissioner for the Food and Drug Administration (FDA), and deputy commissioner Stephen Ostroff, said they were looking for the cause.

“The FDA’s investigators are actively searching for answers as to the source of this outbreak, and what steps can be taken to prevent it from recurring in future growing seasons,” they said.

It had been thought chopped or bagged romaine was the cause, but an Alaskan prison reported several illnesses from whole-head lettuce. Some people who were infected did not eat the lettuce themselves, but had simply been near people who had.

While most E. coli bacteria are not harmful, it can produce toxins that lead to severe illness. Many will recover within a week, but for some the symptoms can be more severe and potentially fatal.

These symptoms can include diarrhea and vomiting, which can also lead to serious ailments such as kidney failure. This can be accompanied by a decrease in the frequency of urination, feeling tired, and losing the pink color from cheeks and eyelids.

All of the tainted lettuce thought to have been produced in Arizona should now have exceeded its shelf life, the CDC noted. The last shipments from the Yuma region were harvested on April 16, with a 21-day shelf life.

“Most of the people who recently became ill ate romaine lettuce when lettuce from the Yuma growing region was likely still available in stores, restaurants, or in peoples’ homes,” it said.

The FDA said that the immediate risk was now gone. But finding the origin of the disease, though, will be of utmost importance in order to prevent a “serious and tragic outbreak” like this happening again.