Federal health officials await test results from California farms and packing plants that could allow them to pinpoint the source of an E. coli outbreak that's sickened spinach eaters across the country.

Though state and federal officials have traced the outbreak to a California company's fresh spinach, they still don't know how bacteria contaminated the leafy greens.

They have ruled out tampering, leaving multiple other potential sources of contamination, including the water and fertilizer that farmers in California's Salinas Valley use to grow much of the nation's spinach crop. Testing could reveal that source, though that isn't guaranteed.

It is the 20th food-poisoning episode since 1995 linked to spinach or lettuce, the Food and Drug Administration said. At least eight were traced to produce grown in the Salinas Valley.

The FDA and the California Department of Health Services again are reviewing irrigation methods, harvest conditions and other practices at farms possibly involved.

For now, officials warn consumers not to eat raw spinach. Natural Selection Foods LLC, whose multiple brands many people reported eating before falling sick, has recalled spinach products distributed throughout the United States. The company also distributed spinach to Canada, Mexico and Taiwan.

Test results on samples from produce packing plants are due in a week or more, said Dr. David Acheson of the FDA's Center for Food Safety and Applied Nutrition. Inspectors were due to begin visiting fields this week.

Various produce growers associations worked with the FDA to publish new guidelines for the safe handling of spinach and other leafy greens in April, after the agency voiced concerns about produce safety.

But the spokesman for a group representing 3,000 growers and shippers in California and Arizona said the new guidelines were not directly in response to any particular incident.

"The basic standard for the industry is zero tolerance," said Tim Chelling, of Western Growers.

One food safety analyst said the Salinas Valley was developing a reputation for food safety problems connected to leafy greens.

"Even the biggest companies have become vulnerable," said Trevor Suslow, a microbial food safety researcher at the University of California, Davis.

Despite the number of spinach and lettuce contamination incidents traced to the Salinas Valley in recent years, California health officials said that could be explained by the sheer volume of crop produced there rather than poor farming practices.

"We don't have specific evidence to suggest that there is a unique issue in the Salinas Valley," Dr. Kevin Reilly, the state Department of Health Services' deputy director of prevention services, said Monday.

E. coli cases linked to tainted spinach have been reported in 21 states: California, Connecticut, Idaho, Illinois, Indiana, Kentucky, Maine, Michigan, Minnesota, Nebraska, New Mexico, Nevada, New York, Ohio, Oregon, Pennsylvania, Utah, Virginia, Washington, Wisconsin and Wyoming, according to the Centers for Disease Control and Prevention.

Wisconsin has reported the most cases, including the death of a 77-year-old woman. A death in Ohio was being investigated.