The investigation into the source of contaminated spinach responsible for at least one death and 109 illnesses across the nation widened today, as federal investigators sought the source of the deadly E. coli bacteria outbreak on individual California farms.

The Food and Drug Administration continued to warn consumers not to eat any fresh spinach or salad blends that include fresh spinach. Illnesses linked to the contaminated spinach have been reported in 19 states; a 77-year-old Wisconsin woman died. Half of the victims are hospitalized, and 15 percent have developed life-threatening kidney failure, according to the Centers for Disease Control and Prevention–a far higher percentage than in a typical E. coli outbreak. "This is unquestionably a significant outbreak," says David Acheson, chief medical officer for the FDA's Center for Food Safety and Applied Nutrition.

Natural Selection Foods LLC, the country's largest grower of organic produce, said late Sunday that the packages of spinach that sickened patients gave health officials were not organic. The company packages both organic and conventionally grown spinach in its San Juan Bautista, Calif., plant. Federal officials have not confirmed the company's assertion. Since late last week, government investigators have focused on Natural Selection's products as a potential source of the outbreak.

The outbreak has left consumers wondering whether any salads are safe. "Normally, the bagged product is the safer product," says Richard Linton, director of the Center for Food Safety Engineering at Purdue University. That's because bagged salads are washed, often put through a sanitizing bath with chlorine, and then bagged, reducing the risk of contamination en route. But, he adds, leafy vegetables are more difficult to wash clean than a smooth-surfaced item like a tomato; a 2005 E. coli outbreak was caused by bagged lettuce.

E. coli contamination almost always comes from cattle, from improperly composed manure fertilizer, contaminated irrigation water, or workers who have touched manure before working with produce.