Officials with the Food and Drug Administration say they may never be able to pinpoint the cause of the recent E.coli outbreak associated with northeastern Taco Bell restaurants. But they believe contaminated lettuce may be the disease source.

Because lettuce is shredded and sold as bulk, tracing back the origin may be difficult, they say.

Officials with the Centers for Disease Control and Prevention say that green onions or onions of any type probably didn't cause the latest outbreak of E. coli food poisoning in the Northeast.

So far, 71 people in five states who have eaten at Taco Bell restaurants have been sickened by the aggressive strain of E. coli 0157, according to the CDC. Of those, 51 were hospitalized and eight developed hemolytic-uremic syndrome, a type of kidney failure.

The states involved are New Jersey, New York, Pennsylvania, Delaware and South Carolina.

Several laboratories tested food samples from Taco Bell restaurants, the CDC reports.

The Food and Drug Administration pulled samples of all foods prepared by Taco Bell. The products include cilantro, cheddar cheese, blended cheese, green onions, yellow onions, tomatoes and lettuce.

On Dec. 6, Taco Bell Corp. announced it was voluntarily removing green onions from its restaurants nationwide. Further testing showed the onions were not the disease source. CDC officials say they still don't know the source and may never.

Taco Bell closed the restaurants where people were sickened and sanitized before reopening them.

The FDA also is working with Taco Bell Corp., its suppliers and distributors to obtain information on sources and product distribution to aid tracing back any products identified as contaminated with the pathogen.