The likely source of an E. coli outbreak in spinach that killed three people and sickened more than 200 was a small cattle ranch about 50 kilometres from California's central coastline, state and federal officials said Friday as they concluded their investigation.

Authorities for the first time said they had isolated the deadly E. coli strain on Paicines Ranch in San Benito County near a field the ranch leased to Mission Organics, a spinach grower.

They found E. coli "indistinguishable from the outbreak strain" in river water, cattle feces, and wild pig feces on the ranch about a kilometre from the spinach fields, the California Department of Health Services and U.S. Food and Drug Administration said in a joint report.

Investigators said they could not make a "definitive determination" as to how the E. coli contaminated the spinach that reached market.

"We'll never be able to make a definitive link, but this shows the inherent risk in the area," said Patti Roberts, a spokeswoman for the health department. "Hopefully that's what good agricultural practices can address to reduce the risk in the future."

The Paicines Ranch, which breeds Angus cattle and quarter horses, said in a statement on its website that it leases land to crop growers and was not under investigation in the outbreak. The ranch could not immediately be reached by phone for further comment.

Otto Kramm, the head of Mission Organics, told The Californian newspaper in Salinas that no pig tracks had been found near spinach fields, although the wild animals are present on the ranch. He said the pigs go through the area to feed on grapes in neighboring vineyards.

Kramm also said he plans to build an $80,000 US fence on the about 182 hectares he leases from Paicines Ranch.

Since the spinach outbreak in September, lawmakers in the U.S. Congress have pushed for regular inspection of processing plants. The FDA announced voluntary guidelines earlier this month for preventing food poisoning in freshly cut produce.