Most oil and gas drillers kept secret at least one chemical used in hydraulic fracturing, an analysis by the Environmental Protection Agency found, raising environmentalists’ concerns about risks from unknown and possibly dangerous ingredients.

The EPA scoured filings on the industry-backed FracFocus online registry and found 11 percent of ingredients used in fracking were deemed a business secret and not disclosed. The EPA said Friday 70 percent of wells examined used at least one chemical that was unidentified.

“The fracking industry is hiding a lot of information about the chemicals they are using in our communities,” Kate Kiely, a spokeswoman for the Natural Resources Defense Council, said of the findings. “Even without that information, it is clear that there is widespread use of dangerouschemicals.”

FracFocus has gained new importance because a rule for fracking on federal lands, released this month, requires drillers to use it to inform the public about the chemical ingredients. Environmental groups complain that the site is cumbersome and gives companies too much leeway to claim confidentiality.

In fracking, or hydraulic fracturing, chemicals are mixed with sand and water and injected underground to free trapped oil and natural gas. The process has spurred a U.S. energy boom, while drawing complaints from residents and health groups that it can contaminate water supplies.

Drilling-service companies say they should, for competitive reasons, be permitted to keep some of their recipe and ingredients secret.

The overwhelming majority of the injected fluid, or 88 percent, is plain water. As much as 7.2 million gallons of water were used for each well fracked, the EPA said. Chemicals used include hydrochloric acid, methanol, ethylene glycol, guar gum and napthalene, according to the report.

The EPA analyzed statements companies posted on FracFocus from early 2011 to early 2013 as part of its study.

The agency had to compile the individual records in the website into a database and then analyze the results. It didn’t study the dangers individual chemicals may pose to human health or the environment.

“As part of our broader assessment, we will focus on toxicity,” said Tom Burke, an official in EPA’s office of research. The amount of chemicals identified by EPA might not reflect the full amount because not all states require disclosure to FracFocus, he said.