Investigators still can't be sure if the E. coli found in cow manure contaminated the fields, but said the find warrants further investigation.
"We do not have a smoking cow at this point," said Dr. Kevin Reilly, deputy director of the Prevention Services Division of the California Department of Health Services. Nevertheless, Reilly called the match an important finding.
There are still many unanswered questions and the probe is continuing, said Dr. Robert Brackett, director of the FDA's Center for Food Safety and Applied Nutrition.
"It's our expectation that no farm should feel they are off the hook," Brackett said.
Investigators still do not know how the feces could have contaminated the spinach implicated in the bacterial outbreak. They also do not know if the ranch used manure from the cattle to fertilize its fields. Nor is there evidence that livestock entered the spinach fields on the ranch. However, wild pigs roamed the property, they said.
"There's lots of wildlife and lots of potential for breakdown in the fencing," Reilly told reporters.
The strain of pathogenic E. coli O157:H7 was found in three cattle fecal samples collected at the ranch, one of four under investigation, the officials said. It matched the strain found in sick patients and in bags of recalled spinach.
Investigators continue to look at agricultural runoff, irrigation water and the hygiene of farm workers as potential sources of the bacteria.
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 investigators have identified a possible source for an E. coli outbreak in the region, Reilly said.
The produce company that processed and packaged the spinach at the center of the outbreak investigation has repeatedly asserted its factories are blameless and pointed to the fields where the greens are grown as the potential source of the problem.
"This definitely reinforces our belief that the source was environmental," said Samantha Cabaluna, a spokeswoman for Natural Selection Foods.