Five people in the United States have died after eating romaine lettuce that was contaminated with E. coli bacteria, whose source remains a mystery, officials said Friday.
A total of 197 people have fallen ill in 35 states, said the US Centers for Disease Control and Prevention.
Since mid-May, "four more deaths were reported, bringing the total to five deaths from Arkansas (1), California (1), Minnesota (2), and New York (1)," the CDC said in a statement.
The outbreak is the largest in the United States since 2006, when spinach tainted with a similar strain of E. coli sickened more than 200 people.
The latest batch of illnesses are believed to be linked to romaine lettuce grown in the Yuma, Arizona region.
However, the investigation continues and the US Food and Drug Administration has said "the illnesses associated with this outbreak cannot be explained by a single grower, harvester, processor, or distributor."
Romaine lettuce grown in the Arizona region was last harvested in mid-April.
"It is unlikely that any romaine lettuce from the Yuma growing region is still available in stores or restaurants, due to its 21-day shelf life," said the FDA.
Eating the contaminated lettuce may cause diarrhea, vomiting and even kidney failure in severe cases.
"Most of the newly reported cases are people who became sick two to three weeks ago, still within the window when contaminated romaine was available for sale," said the CDC statement.
"Some people who became sick did not report eating romaine lettuce, but had close contact with someone else who got sick from eating romaine lettuce."
Nearly half of those who became ill had to be hospitalized. A total of 26 people have developed a type of kidney failure called hemolytic uremic syndrome.