Multiple E. coli outbreaks late last year that sickened nearly 200 people who ate California-grown romaine lettuce were likely due to cattle operations near the fields, the Food and Drug Administration (FDA) has said.

In a report, the FDA said that feces from the cows are considered "the most likely contributing factor" to three outbreaks in November and December traced to farms in the Salinas Valley.

The feces, which contain the bacteria, could have been spread by water run-off, irrigation water, wind, animals or vehicles, it said.

However, the agency did not say that cows were the definitive source of the outbreaks, which affected people in at least 16 states and Canada. No deaths were reported.

Investigators concluded that the illness was centered on ranches and fields owned by the same grower and that were located downslope from public land where cattle grazed.

"FDA considers adjacent or nearby land use for cattle grazing as the most likely contributing factor associated with these three outbreaks," it said in the report.

"Agricultural water sources used to grow the romaine" also were possible routes, the report’s executive summary said.

Another E. coli outbreak in spring 2018 that sickened more than 200 people and killed five was traced to tainted irrigation water near a cattle lot.

Between 2009 and 2018, federal authorities identified 40 food-borne outbreaks of E. coli in the U.S. "with a confirmed or suspected link to leafy greens," the FDA said.