Results from the Food and Drug Administration's (FDA) Total Diet Study between 1995 and 2001 show a powdered fruit drink and a non-diet cola drink containing benzene almost 10 and 14 times above the World Health Organisation's 10 parts per billion limit for benzene in tap water.
The results, which also showed average benzene levels above the FDA's five parts per billion limit in some drinks varieties, were uncovered by a US campaigns body, the Environmental Working Group.
The FDA, however, has now said the data was "inconsistent" with other tests for benzene in soft drinks, and that an investigation suggested "elevated benzene levels can be formed by the procedures used to analyse beverage samples".
Benzene is a known carcinogen, and renewed concerns about its presence in soft drinks have grown since an FDA scientist revealed to BeverageDaily.com in February that recent tests had again found some soft drinks with benzene above the US water limit.
An FDA spokesperson has again confirmed this to BeverageDaily.com, adding that the agency was engaging with the relevant soft drinks firms to minimise or eliminate benzene formation in drinks.
The suspected source of the benzene is two common ingredients in the drink - sodium benzoate and ascorbic acid (vitamin C).
Both the FDA and US soft drinks industry have known this for 15 years, but hatched a deal for the industry to spread the word and reformulate privately, a BeverageDaily.com investigation learnt earlier this year.
The FDA's announcement this week was its first public statement on the issue, although it refused to publish results from its recent testing, saying the investigation was not yet complete.
The statement follows a letter to the FDA from two US Congress members, who demanded the agency explains what it knows about benzene in soft drinks, including action is has taken in the past and intends to take now.
They criticised the FDA's decision in 1990 to trust the industry to get the word out and reformulate, adding this had "amounted to nothing more than empty promises".
An FDA scientist there at the time told BeverageDaily.com a follow-up survey of soft drinks in 1993 found no problem with benzene.
And, an agency spokesperson said the majority of drinks tested recently contained either no benzene or levels under the US water limit. She re-iterated that none of the levels found so far were considered a health risk for consumers.
Several questions, however, remained unanswered by the FDA.
It was unclear whether the agency was using the five parts per billion limit for benzene in drinking water as applicable to soft drinks. There is no specific limit set for soft drinks.
The agency also declined to comment on what had been done to follow through on elevated benzene levels found in drinks during its Total Diet Study between 1995 and 2001.
Glen Lawrence, a scientist who helped the FDA work out the link between benzene and the ascorbic acid-sodium benzoate combination in drinks in 1990, was unsure about the agency's explanation that testing procedures were to blame for higher benzene levels during the Total Diet Study.
"It is possible, but probably is not important in the beverage analyses," he said.
Industry testing in 1990 showed that benzene levels were likely to increase in drinks exposed to higher temperatures during transport and storage.
Kevin Keane, of the American Beverage Association (ABA), assured consumers there was no health risk, but said some brands may not be aware of the potential for sodium benzoate and ascorbic acid to form benzene.
"15 years ago it was under control, but this is a fast-growing industry. There are a lot of new companies, a lot of new brands and things have changed," he told BeverageDaily.com in February.
US lawyers this week filed class action lawsuits against two soft drinks firms - In Zone Brands, who make Bellywahsers drinks, and Polar Beverages - alleging independent lab tests showed they had drinks contaminated with benzene above the US water limit.