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 Central Valley.

The tainted spinach, grown in California's Central Valley, has sickened 104 people and killed 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 on an as-needed basis 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 cannot perform the tests or when a second opinion is needed, Russell said. She said the center will remain open until the crisis abates.

E. coli cases linked to tainted spinach have been reported in 19 states, with a majority of cases in Wisconsin. Only one illness has been reported in California.

The Food and Drug Administration continued to warn consumers this weekend to avoid spinach.

"We're still in the process of alerting people of the fact that they need to check the refrigerator," Russell said.

The spinach could have been contaminated in the field or during processing. About 74 percent of the fresh market spinach grown in the U.S. comes from California, according to the California Farm Bureau Federation.

There have been previous bacterial contamination outbreaks linked to spinach and lettuce grown in the state.

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.

Federal officials stressed that the bacteria had not been isolated in products sold by the holding company, based in San Juan Bautista, California, and known for Earthbound Farm and other brands. As the investigation continues, other brands may be implicated, officials said.

Natural Selection officials could not be immediately reached on Sunday.

The company was founded in 1984 by Drew and Myra Goodman, who left Manhattan to farm a small raspberry farm in Californias Carmel Valley. Within two years, Earthbound Farm began shipping pre-washed, packaged salad fixings, and the company's "spring mix" became a mainstay of restaurants and supermarkets.