The strain of pathogenic E. coli O157:H7 was found in three cattle fecal samples collected at one of four ranches under investigation, the officials said. The ranch is within a mile of produce fields.
"We do not have a definite cause-and-effect here. We do have a very important finding," said Dr. Kevin Reilly, deputy director of the Prevention Services Division of the California Department of Health Services.
Investigators still do not know how the feces could have contaminated the spinach implicated in the bacterial outbreak.
The recent outbreak of E. coli in spinach killed three people and sickened nearly 200 in half the states. It was the 20th such outbreak in lettuce or spinach since 1995.
The find marks the first time that a strain of E. coli implicated in an outbreak in the produce-growing region has been linked to a plausible source, Reilly said.
But Reilly said investigators don't have a "smoking cow" and cannot say for certain whether the cattle tested were indeed the source of the contamination.
Nor did investigators know if the ranch used manure from the cattle to fertilize its fields. Wildlife wandering the ranch also could have spread the bacteria. The probe is ongoing, said Dr. Robert Brackett, director of the FDA's Center for Food Safety and Applied Nutrition.