Investigators considered cheddar cheese and ground beef as possible contaminated ingredients before settling on lettuce. The company first believed green onions were responsible, but follow-up testing by the government failed to confirm that.
Interviews with patients and other investigative work pointed toward lettuce as the culprit.
"That I would say is the most likely vehicle. I would warn we are not done with the investigation," Dr. Christopher Braden, a medical epidemiologist with the Centers for Disease Control and Prevention, told reporters.
Taco Bell's menu, with its various combinations of the same ingredients, made it difficult to pinpoint the source of the contamination.
"That has been the case and is part of the reason it has taken a number of days to identify what might be the contaminated ingredient," Braden said.
Dr. David Acheson, chief medical officer at the Food and Drug Administration's Center for Food Safety and Applied Nutrition discussed plans to trace the lettuce to its source. But because the shredded lettuce was processed in bulk, that could turn out to be a hard task.
The evidence that lettuce was responsible for the illnesses was statistical — it was the item that victims most commonly reported eating.
Taco Bell's president, Greg Creed, said the CDC and FDA told the company "it was highly unlikely it's cheese or beef because cheese is pasteurized and the beef preparation ruled them out."
He declined to identify the supplier of the lettuce to the Northeastern restaurants, said 80 percent of the lettuce from that supplier did not go to Taco Bell and noted Taco Bell has a new supplier for those states.
The lettuce potentially linked to the outbreak came from the West, he said, declining to be more specific.
Acheson said no Taco Bell food samples, other than white onions from a New York restaurant, have tested positive for E. coli, Acheson said. The E. coli found in the white onion did not match the strain that sickened Taco Bell customers, however.
He said there is no evidence the Taco Bell outbreak is connected to cases of illnesses linked to Taco John restaurants in Iowa and Minnesota.
Health officials in those states said the cause of the outbreak — which has sickened at least 50 people in Iowa and 27 in Minnesota — has been tied to lettuce served at Taco John's in three cities.
Most E. coli infections are associated with undercooked meat, but increasingly produce is to blame. This fall, an outbreak linked to fresh spinach killed three people and sickened more than 200 others.
"This is a situation that is not tenable," Braden said.
Health officials believe most cases of E. coli contamination originate on the farm, where produce can come into contact with animal feces laden with the bacteria.
E. coli, or Escherichia coli, is a common and ordinarily harmless bacteria found in the guts of cattle and other animals. The E. coli O157:H7 strain can cause abdominal cramps, fever, bloody diarrhea, kidney failure, blindness, paralysis, even death.