Contaminated irrigation water may have contributed to the largest outbreak of Salmonella in over a decade linked to red onions, FDA revealed Thursday.

The Salmonella Newport outbreak investigated last year by federal officials was associated with 1,127 U.S. illnesses, as well as 515 cases in Canada.

FDA during its probe identified several factors that may have contributed to the outbreak—outlined in a report made public—including tainted irrigation water.

Other possible sources of contamination included “sheep grazing on adjacent land, signs of animal intrusion including scat (fecal droppings) and large flocks of birds that may spread contamination, as well as packing house cleaning and sanitizing practices,” said Frank Yiannas, FDA’s deputy commissioner for food policy and response, in a statement.

FDA’s investigation identified Bakersfield, California-based Thomson International Inc. as the likely source of contaminated red onions. As a raw agricultural commodity, whole red onions had not previously been associated with an outbreak of foodborne illness, according to FDA.

While FDA did not identify a “conclusive root cause” of the Salmonella outbreak, the agency’s “leading hypothesis is that contaminated irrigation water used in a growing field in Holtville, California may have led to contamination of the onions,” Yiannas shared. “Considering these findings, the FDA encourages all produce growers to assess risks that may be posed by adjacent and nearby land uses, especially as it relates to the presence of livestock and the interface between farmland, rangeland, irrigation water and other agricultural areas.”

Thomson International could not be immediately reached for comment through a phone number provided in a 2020 recall notice associated with the outbreak.

Salmonella each year causes about 1.35 million infections, 26,500 hospitalizations and 420 deaths in the U.S, according to estimates from the Centers for Disease Control and Prevention (CDC). Most people who fall ill from Salmonella suffer diarrhea, fever and stomach cramps, but they recover on their own without specific treatment, according to the CDC.

State, federal and Canadian officials investigated the outbreak of Salmonella Newport linked to onions. In the U.S., the CDC tied the outbreak to more than 1,100 reported illnesses in 48 states.

No deaths were associated with the outbreak. CDC, however, identified 167 hospitalizations.

“FDA recognizes the interconnection between people, animals, plants and their shared environment when it comes to public health outcomes,” FDA stated in the investigation report, “and we encourage collaboration among various groups in the broader agricultural community (e.g., produce growers, those managing animal operations, state and federal government agencies, and academia) to address this issue.”