SOUTH PLAINFIELD, N.J. -- The company that washed, chopped and packed the green onions suspected in an E. coli outbreak that has sickened nearly four dozen people on the East Coast bought its scallions from a California grower, company officials said Thursday.

Ready Pac Produce, the sole supplier of green onions to Taco Bell restaurants in the New York, New Jersey and Philadelphia areas, stopped all production of scallions at its Florence, N.J. plant, which was visited Wednesday by federal food inspectors.

The scallions processed there were grown in California, said Steve Dickstein, marketing vice president for Irwindale-based Ready Pac, one of the nation's leading produce packers.

"The source of the growing region was California," Dickstein said. "So as soon as we heard news from Taco Bell about the positive yet inconclusive results, we took immediate action to do everything we could."

It was not immediately clear where in California the green onions were grown, but Boskovich Farms Inc. of Oxnard, which supplies green onions to Taco Bell, said it was working with the fast-food retailer in the efforts it was taking after the outbreak.

"Boskovich was the source of the onions, although the investigation continues as to whether they were contaminated," Dickstein said.

The farm did not return repeated calls for comment. Its Web site said it had not been contacted by health officials.

This is the second E. coli scare to hit Ready Pac in the past four months. In September, spinach with the Ready Pac label was among dozens of brands pulled from the shelves when federal authorities traced a nationwide E. coli outbreak to a San Juan Bautista processing plant that bags its spinach and dozens of other brands.

Green onions have been the source of nine food-borne illness outbreaks since 1994, said Douglas Powell, food safety professor at Kansas State University.

If scallions are contaminated at any stage of the growing process, their structure makes it difficult to remove the bacteria solely by washing because the onions can carry pathogens inside their multiple layers, Powell said.

Ready Pac, which won a supplier award from Taco Bell in 2001, washes and sanitizes its onions two times, Dickstein said.

Taco Bell ordered the removal of green onions from its 5,800 restaurants nationwide on Wednesday after preliminary testing by an independent lab found three samples of green onions appeared to have a harsh strain of the bacteria.

The U.S. Food and Drug Administration said it planned to also test cilantro, cheddar cheese, blended cheese, yellow onions, tomatoes and lettuce for traces of the bacteria.

Ready Pac has not halted production of the other vegetables it supplies to Taco Bell, Dickstein said.