Health investigators in the US have found spinach contaminated with E.coli in the refrigerator of a person made ill by the bacteria.
Packaged spinach has been removed from US supermarkets
Officials said the discovery could be the "smoking gun" that leads to a source of the 23-state outbreak.
The contaminated spinach, found in New Mexico, was produced in one of three counties in California, tests showed.
One woman has died and 146 other people have become ill during the outbreak, leading to a nationwide spinach ban.
The bag of Dole baby spinach was found in the refrigerator of a woman ill with E.coli-related symptoms, officials said.
Narrowing the options
Authorities said the discovery would help them track the spinach back to its source, increasing the chances of stopping the spread of the bacteria.
Dr David Acheson of the Food and Drug Administration, the body responsible for national food safety standards, said finding the contaminated spinach was a breakthrough.
"Yesterday we had it down to California. Today we've got it down to three counties," he said.
"We want it down to a salad bowl and eventually a spinach leaf."
Efforts to find the source of the rogue spinach - believed to have been contaminated by animal faeces - were concentrating on nine farms in California's Salinas Valley, a state official said.
The area produces most of California's spinach, while the state itself supplies some 75% of the entire US spinach crop.
Infection by the E.coli bacteria causes diarrhoea and possible kidney failure.
Normally found in the intestines of people or cattle, it is particularly harmful to young children and pensioners.