California officials have discovered E. coli in cattle feces on pastures near farms being investigated as possible sources of spinach contaminated with the bacteria, which caused a nationwide outbreak of food poisoning.

Reuters reports that state investigators have obtained eight samples of cattle feces testing positive for E. coli. The samples are being retested to see if their bacteria strain matches the strain in 193 cases of food poisoning, including one confirmed fatality last month, all linked to tainted spinach.

The bacteria could have been transmitted to the spinach by irrigation water, fertilizer, farm equipment or workers, livestock and wild animals, or it could have contaminated the spinach inside processing plants, during transport or on store shelves.

The U.S. Food and Drug Administration has said that fresh spinach is safe to eat in the United States because all E. coli-tainted spinach had been recalled.