But an official representing McLane Foodservice Inc. said Wednesday that it's highly improbable the company was the source of the contamination that affected people in New Jersey, New York and Pennsylvania.

"A truck backs up to our dock with cardboard boxes filled with [bagged] sliced green onions," Mr. McKay said in a telephone interview. "We unload the boxes, stack them temporarily ... then ship them out to the restaurants. We don't touch food in any way."

On Wednesday, Taco Bell announced it would remove green onions from its 5,800 U.S. restaurants after an independent laboratory found three samples to be "presumptive positive" for E. coli O157:H7, the most dangerous strain of the bacterium.

"In an abundance of caution, we've decided to pull all green onions from our restaurants until we know conclusively whether they are the cause of the E. coli outbreak," said Greg Creed, Taco Bell's president, in a prepared statement. "Taco Bell's first concern is the health and safety of our customers and employees."

The outbreak also prompted Yum Brands Inc. to temporarily close multiple restaurants. The company, based in Louisville, Ky., owns Taco Bell, Pizza Hut and other fast-food chains