Lettuce was the most likely source of an outbreak of E. coli linked to Taco Bell, federal health officials said Wednesday.

Taco Bell had said contaminated green onions were responsible for the cases of food poisoning — 71 confirmed cases of E. coli in five states, primarily New Jersey, New York and Pennsylvania, as of Wednesday, according to the Centers for Disease Control and Prevention.

Follow-up government testing, however, failed to confirm that.

Interviews with patients and other work led them to believe that lettuce was the probable culprit, health officials said.

"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 CDC, told reporters.

Investigators had considered cheddar cheese and ground beef as well.