WASHINGTON - The Environmental Protection Agency subpoenaed energy giant Halliburton today, seeking a description of the chemical components used in a drilling technique called hydraulic fracturing.

The EPA said it issued the subpoena after Texas-based Halliburton refused to voluntarily disclose the chemicals used in the controversial drilling practice, also known as “fracking.” Halliburton was the only one of nine major energy companies that refused the EPA’s request.

The agency said the information is important to its study of fracking, in which crews inject vast quantities of water, sand and chemicals underground to force open channels in sand and rock formations so oil and natural gas will flow.

The EPA is studying whether the practice affects drinking water and the public health.