The Environmental Protection Agency sent letters to nine drilling companies on Thursday requesting detailed information about the chemicals contained in fluids used to crack open underground rock formations in the hunt for oil and natural gas.

The move is part of the federal agency’s preparations for a long-term scientific study of the effects of the practice, known as hydraulic fracturing or “fracking,” on drinking water and public health.

“Natural gas is an important part of our nation’s energy future, and it’s critical that the extraction of this valuable natural resource does not come at the expense of safe water and healthy communities,” the E.P.A. administrator, Lisa P. Jackson, said in a statement.

The agency asked the companies to respond to its request within seven days and to voluntarily provide the information within 30 days, according to a copy of the letter provided by the agency.