The U.S. Department of Agriculture said late Wednesday that spring onions were the likely source of the outbreak.

Past E. coli outbreaks have hampered sales at major fast food restaurants as customers stayed away from affected chains out of fear of the disease. Regulators are still investigating whether McDonald’s beef patties are affected, but while proper cooking kills E. coli in beef, McDonald’s Quarter Pounder is served with raw, sliced ​​onions.

McDonald’s has withdrawn the Quarter Pounder from about one-fifth of its U.S. restaurants in Colorado, Kansas, Utah and Wyoming, as well as parts of Idaho, Iowa, Missouri, Montana, Nebraska, Nevada, New Mexico and Oklahoma.

“We have been told by the agency not to use onions for the foreseeable future,” Maria Gonzales, on-duty manager at Burger King in Longmont, Colorado, said Wednesday. he said. “They are off our menu.”

McDonald’s did not immediately respond to a request for comment Thursday.

McDonald’s is moving quickly to contain the damage while also trying to reassure customers of its efforts. This can be critical; Previous outbreaks at Chipotle Mexican Grill in 2015 and Jack in the Box in 1993 caused those companies’ sales to drop sharply for several quarters.

David Tarantino, an analyst at Baird Equity Research, downgraded McDonald’s shares to “neutral” late Wednesday. “We are concerned that reports of an E. coli outbreak linked to McDonald’s restaurants in several US states could pose a major threat to consumer confidence and therefore harm comparable store sales in the US,” he said.