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.

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.

The Associated Press reports that 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.

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.

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.