Federal testing has failed to confirm that scallions caused an outbreak of E. coli that sickened 64 people who ate in Taco Bell fast-food restaurants in the northeastern United States, health officials said Monday.

During the weekend, Taco Bell officials said they determined that the green onions were the likely source of the bacteria, but follow-up federal testing of those samples was negative for E. coli.

"In that context, we have not ruled out any food items," said Dr. David Acheson, chief medical officer for the Food and Drug Administration's Center for Food Safety and Applied Nutrition.

Meanwhile, health officials in New York state said a sample of white onions taken from a Taco Bell restaurant tested positive for E. coli, but the strain has not been linked to any cases of illness in the United States during the past 30 days. The positive sample initially was mistakenly identified as being green onion, Acheson said.

The Centers for Disease Control and Prevention have confirmed 64 E. coli cases in five states, mostly in New Jersey, New York and Pennsylvania. Two were found in Delaware and one in South Carolina. The South Carolina patient had eaten at a Pennsylvania Taco Bell.

Meanwhile, nearly three dozen people have fallen ill with symptoms consistent with E. coli infection after eating at a Taco John's restaurant in Cedar Falls, Iowa.

There is no indication the outbreaks are linked, but the CDC has not ruled out a connection, said Dr. Christopher Braden, a medical epidemiologist with the agency. Iowa officials suspect a sick restaurant worker could have spread the illness. The two restaurant chains are not related.

It could take until midweek to determine whether the Taco Bell outbreak is over, Braden said. However, there have been no reports of anyone falling ill since Dec. 2, he added.