Government and other experts say that water on the farms could be a likely culprit in the Salinas Valley farms where E. coli has contaminated the spinach crops, though that has not been proved.

Salinas Valley is believed to be the source of at least eight E. coli outbreaks since 1995. Twice in the past two years, federal health officials told California growers of lettuce and other leafy greens, mostly in Salinas Valley, to make their farming practices safer.

Floodwaters from nearby creeks and rivers are being suspected now as it was when the letter was sent by health officials to the California growers. The letter said that although it was unlikely that contamination in all of the outbreaks was caused by flooding from agricultural water sources, farmers should dispose of any ready-to-eat produce that comes in contact with floodwaters because the produce could be exposed to sewage, animal waste, heavy metals, pathogenic microorganisms or other contaminants.

While investigators warned against reaching premature conclusions, experts said from everything they knew of the current outbreak, most signs pointed to E. coli-contaminated water. Proving it may be very difficult, although experts have long felt irrigation water may be the source of the repeated outbreaks.

“Contaminated irrigation water is one of the most common vehicles by which E. coli may be introduced into the environment,” concluded Abasiofiok Ibekwe of University of California-Riverside in his 2005 paper in the journal Environmental Science and Technology. He found that the microbe can persist for more than 45 days in soil.

Well-water contamination in Salinas Valley and elsewhere has been blamed for at least two other major outbreaks of disease due to produce contamination. In one of these cases, the well had not been shielded at the surface. The cause of bad well water was not identified in the second case.

In a third major outbreak, investigators found that lettuce fields had been flooded by water contaminated by cattle grazing in an adjacent field.