The Environmental Protection Agency has asked nine drilling companies to provide detailed information about the chemicals they use in a controversial process for extracting natural gas.

The technique, known as hydraulic fracturing or “fracking”, involves breaking up underground rock formations to release gas, and is being investigated by the EPA following concerns raised by Congress and the public about its potential effects on public health and drinking water.

The EPA asked the companies to reveal the chemical composition of fluids that are mixed with water and sand, then injected at high pressure underground. Drillers say the process is safe, but up to now have declined to disclose the chemical formulas, claiming they are trade secrets.

The agency also asked the companies–including Halliburton, Schlumberger and Superior Well Services–to provide data on the impact of fracking fluids on health and the environment.

“By sharing information about the chemicals and methods they are using, these companies will help us make a thorough and efficient review of hydraulic fracturing and determine the best path forward,” EPA Administrator Lisa P. Jackson said in a prepared statement. The agency said it was asking the companies to provide the data voluntarily, but is “prepared to use its authorities to require the information” should they refuse.

Partly because of the secrecy surrounding the chemical compounds, environmental and community groups have demanded greater scrutiny of fracking, saying it threatens to contaminate drinking water and has not been proven safe. The EPA essentially endorsed the safety of the process in a 2004 study, but critics said the study was sloppy and more research was needed. In March, the EPA agreed to conduct an extensive review.

In remarks to The New York Times, Stephanie Meadows, a senior policy adviser for the American Petroleum Institute, a trade group representing oil and natural gas companies, criticized the agency’s threat to take legal action to compel the disclosures.

“I’m not sure how they would do that, or if they even have the authority to do that,” she said. “I’m [a] little disappointed with the threats, because I thought we’d made it clear all along that we want to be helpful.”

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Most companies that make the fluids used in hydraulic fracturing have declined to disclose their formulas, arguing that the exact components are trade secrets.