Five people have died and nearly 200 people from nearly three dozen states have been sickened by E. coli in a growing outbreak that has so far stumped federal investigators.

The Centers for Disease Control and Prevention announced the death tally Friday, more than two months after the first illnesses occurred in mid-March. Although investigators have determined that E. coli came from contaminated romaine lettuce that were grown in Arizona’s Yuma region near the border to Southern California, the Food and Drug Administration has not been able to link the outbreak to one farm, processor or distributor, Scott Gottlieb, the agency’s commissioner, and Stephen Ostroff, the deputy commissioner for foods and veterinary medicine, said in an update Thursday.

With the tainted vegetables now off the shelves and the growing season over, the FDA may never crack the case, frustrating consumer advocates who have called on the agency to issue rules that would speed up future investigations of foodborne illnesses, The Washington Post’s Caitlin Dewey reported.

The CDC said 197 people have been sickened, nearly half of whom were hospitalized. Some told officials that they did not eat romaine lettuce but became sick after close contact with someone who ate contaminated vegetables, the CDC said. Twenty-six developed hemolytic uremic syndrome, a type of kidney failure that can be life-threatening to people with weak immune systems, such as young children and the elderly.

A majority of those who have been affected come from California, where one death was reported, and Pennsylvania. The four other deaths were reported in Arkansas, Minnesota and New York, according to the CDC.

Last month, the Canadian government announced that six of its citizens were also sickened with Escherichia coli with “similar genetic fingerprint” with those reported in the United States. Two of the six told officials that traveled across the border before they became sick. Three became infected in Canada. Canadian officials, though, said that the risk to their citizens are low.

The FDA initially said that only bagged and pre-chopped romaine lettuce that have been distributed to retailers across the country were contaminated with E. coli, but a group of inmates at a prison in Alaska also became sick after eating whole-head lettuce.

Because majority of the illnesses came from prepackaged vegetables that have been passed on from suppliers to distributors to processing facilities where they were chopped and bagged, finding out where they were grown is far more cumbersome.

“It’s a labor-intensive task. It requires collecting and evaluating thousands of records; and trying to accurately reproduce how the contaminated lettuce moved through the food supply chain to grocery stores, restaurants and other locations where it was sold or served to the consumers who became ill,” Gottlieb and Ostroff said in their joint statement.

This is the worst outbreak since 2006, when 205 people became ill and five died from E. coli from baby spinach.

E. coli are a type of bacteria found in undercooked beef, raw milk, soft cheeses made from raw milk, raw fruits and vegetables, and contaminated water. Most E. coli bacteria are not harmful, but one type known as E. coli O157: H7 produces a toxin called Shiga, which destroys red blood cells, causes kidney failure and bloody diarrhea. Other kinds of E. coli cause urinary tract infections, respiratory illness and pneumonia.

Health officials said children under the age of 5, seniors older than 65 and those with weak immune systems are most vulnerable. So far, the outbreak has sickened people ages 12 to 84.