SAN FRANCISCO – Federal agents from the Centers for Disease Control and Prevention intervened this weekend to help investigate an E. coli outbreak tied to tainted spinach from California’s Salinas Valley.

The greens, which appear to be grown by the world’s largest producer of organic produce, have sickened 109 people and caused the death of a 77-year-old woman, according to health officials.

CDC officials said Sunday they have started an Atlanta-based emergency operations center to help state health agencies with E. coli testing. Epidemiologists are helping test spinach samples and stool samples of those who have been infected, center spokeswoman Lola Russell said.

The center is helping when state health agencies can’t perform the tests or when a second opinion is needed, Russell said.

E. coli cases linked to tainted spinach have been reported in 19 states with a majority of cases in Wisconsin.

Other states reporting cases were California, Connecticut, Idaho, Indiana, Kentucky, Maine, Michigan, Minnesota, New Mexico, Nevada, New York, Ohio, Oregon, Pennsylvania, Utah, Virginia, Washington and Wyoming, according to the CDC.

The Food and Drug Administration continued to warn consumers not to eat fresh spinach or fresh spinach-containing products until further notice.

Natural Selection Foods LLC recalled its packaged spinach throughout the United States, Canada and Mexico as a precaution after federal health officials said some of those hospitalized reported eating brands of prepackaged spinach distributed by the company.