Even so, the FDA still believes there are, or sodas, said Laura Tarantino, the agency's director of food additive safety.
"We haven't changed our view that right now, there is not a safety concern, not a public health concern," she said Wednesday. "But what we need to do is understand how benzene forms and to ensure the industry is doing everything to avoid those circumstances."
The admission contradicted statements last week, when officials said FDA found insignificant levels of benzene.
In fact, a different study found benzene at four times the tap water limit, on average, in 19 of 24 samples of diet soda.
Tarantino said chemists may have overestimated the amount of benzene and that levels in diet soda were still relatively low compared with other sources of benzene exposure.
The samples were collected as part of the FDA's ongoing Total Diet Study, which looks for contaminants and nutrients in many foods and beverages.
FDA has been doing a separate study of benzene in soft drinks, but it is not ready to release the results, Tarantino said.
The Environmental Working Group has accused the FDA of suppressing information about benzene in soft drinks.
"If they're so confident the situation is not a safety risk, they need to release the data to prove it," said Richard Wiles, the group's senior vice president. "The only data available to the public contradict their claim."
Benzene, a cancer-causing chemical linked to leukemia, can form naturally and is found in forest fires, gasoline and cigarette smoke. It is widely used in industrial production to make plastics, rubber, detergents, drugs and pesticides.
Benzene can also form in soft drinks made with Vitamin C and sodium or potassium benzoate. Heat, light and shelf life can affect whether benzene will form, according to FDA.
A spokesman for the American Beverage Association said the amount of soft drinks people consume is far less than the amount of tap water they are exposed to.
"You can crunch the numbers any way you want; it's still adding up to safe products," said the spokesman, Kevin Keane. "We're going to continue to work with FDA to ensure the safety of our products."
By Libby Quaid