Salinas ValleyThe Chicago Tribune reports that sunny Salinas Valley holds a dark mystery: Why, in the past decade, have nine E. coli outbreaks been linked to produce grown here?

It’s still unknown why this fertile land has been hit by what an FDA official calls "significant" crop contamination. On Sunday, officials announced that the number of people sickened by an E. coli outbreak linked to Salinas Valley spinach has risen to 109 in 19 states.

A Wisconsin woman has died. Natural Selection Foods, which grows spinach in the Salinas Valley, has recalled the leafy green under 34 brands. On Sunday, federal authorities identified a second producer linked to the contamination, River Ranch of California, because it had bought spinach from the first grower. River Ranch on Sunday recalled spring mixes containing spinach with the labels Hy Vee, Fresh and Easy, and Farmers Market.

Throughout the picturesque terrain here, questions swirl.

Has cattle waste contaminated irrigation water? Does contaminated soil blow in the wind? Do birds feeding on cow manure carry E. coli? Is such contamination inevitable to farms so big that the area is known as "The Salad Bowl" to the nation and world?

Over 10 years, E. coli outbreaks linked to produce grown here have sickened more than 300 people and killed three. Not even federal or state authorities know why.

"That’s a very good question, and we don’t have an answer to that," said Dr. David Acheson, chief medical officer for the Food and Drug Administration’s Center for Food Safety and Applied Nutrition. "Certainly huge amounts [of vegetables] are produced there, which is clearly a factor."

What’s certain is the deadly virulence of the E. coli, and together federal and California officials in August began investigating how California lettuce has been contaminated. On Monday, federal investigators will visit an undisclosed number of Salinas Valley farms, using a "trace back" technique to identify the ultimate source of the food-borne poisoning, an official said Sunday. On the streets and in the fields, much local speculation has focused on water.