WASHINGTON (CNN) -- A nationwide outbreak of E. coli poisoning that sickened at least 205 people has been traced to pre-packaged spinach from a single California field, processed on a single day last summer, according to a report released Friday.
But exactly how that field became contaminated is impossible to say, the report said.
The report found several possible causes of the contamination - including tainted water, cow feces in a nearby field, and wild pigs roaming in the area - but said it's impossible to definitively say which was at fault.
The outbreak last summer caused illnesses in 26 states and led to three deaths.
It ignited a six-month investigation by California's Department of Health Services (CADHS) and the U.S. Food and Drug Administration.
Based on interviews with victims, investigators last September traced the sicknesses to pre-packaged spinach processed by Natural Selection Foods in San Juan Bautista, Calif., and sold as baby spinach under the Dole brand name.
"The probe was a notable effort by federal, state and local officials," said Robert E. Brackett, director of the FDA's Center for Food Safety and Applied Nutrition. "It yielded valuable information we can use to determine how best to reduce the likelihood of similar outbreaks."
Investigators found the tainted spinach was packaged on August 15 and had come from four nearby growing operations.
Dr. Kevin Reilly, deputy director of prevention services at CADHS, said investigators took more than 850 environmental samples from the Paicines, Wickstrom, Taix and Eade ranches in Monterey County and San Benito County, finding the deadly O157 strain of E. coli in river water, cow feces and wild pigs.
Friday's report says that all the samples testing positive for O157 were associated with a single farming operation, Mission Organics, which leased fields on the Paicines Ranch.
Because there were so many possible sources of contamination, the report had to conclude that "no definitive determination could be made regarding how E. coli O157 pathogens contaminated spinach in this outbreak."
"It takes a single breakdown," Reilly said. "But today, we're better than we were a year ago. ... We will continue to improve safety associated with leafy green vegetables."
Natural Selection Foods has already taken some steps to improve safety, such as using two mobile laboratories to test for E. coli in produce entering the plant.
But critics of the industry say that growers or the government could do more, such as regularly testing all water supplies and requiring at least several hundred yards of separation between cattle and growing operations.
And California State Sen. Dean Florez, D-Shafter, said the report makes the case for "mandatory" regulations that can be enforced - with fines levied.
"The report is devoid of any action plan on how their recommendations should or will be implemented or by whom," Florez said.
"I think it is very shameful that our state government, especially the agencies that are responsible for the health and welfare of the public, have relegated themselves as simple spectators to the most deadly E. coli outbreak emanating from California."
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