Santa Ana, Calif ? Federal authorities are focusing on shredded lettuce as the possible food source responsible for causing severe illnesses of 71 people who ate at Taco Bell restaurants in several states.

Though the E. coli strain has not been found in lettuce – or any other food samples taken from Taco Bell eateries – health officials said Wednesday that lettuce “was the most likely source of the outbreak.”

The latest findings were based on eating patterns of the victims.

The Centers for Disease Control and Prevention said cheddar cheese and ground beef were also common ingredients eaten by those who got sick. However, “what we found is the most likely food vehicle is lettuce,” said Dr. Christopher Braden, medical epidemiologist for the CDC, the agency handling the case study of victims.

Irvine, Calif.-based Taco Bell said late Wednesday it was not going to remove lettuce from its menu.

“It’s not necessary for us to remove lettuce,” said Taco Bell president Greg Creed, citing the company’s decision to change produce suppliers in the Northeast.

Lettuce is in about 70 percent of Taco Bell’s products, Creed said.

Taco Bell said the lettuce supplied to its restaurants in New York, New Jersey, Pennsylvania and Delaware was grown by various farmers and shipped to the chain’s former produce supplier.

It did not name the supplier.

This week, Taco Bell switched its East Coast produce supplier to Taylor Farms as a “strictly precautionary measure,” the company said.

Despite the growing number of cases tied to Taco Bell, the federal agencies, as well as the fast food chain, said they believe the outbreak has been isolated to the East Coast.

“All the lettuce sold in these restaurants today is from a different source, and anything sold prior to Dec. 3, the last date of illness related to Taco Bell, has long been sold or discarded,” Taco Bell said in a statement.

The nation’s largest Mexican fast food chain was first notified two weeks ago that diners eating at its New York and New Jersey locations had fallen ill. More victims have turned up in other states, prompting a federal investigation.