Cuadrilla Resources will start fracking in the UK. Reuters

Halliburton Company and Baker Hughes Inc. said the U.S. Justice Department is reviewing antitrust violations against the companies' hydraulic fracturing services, Bloomberg reported Friday.

The Justice Department confirmed its investigation but did not provide details.

“The Antitrust Division is investigating the possibility of anticompetitive practices involving pressure-pumping services performed on oil and gas wells,” said Gina Talamona, a spokeswoman for the department’s antitrust division.

Halliburton (NYSE:HAL) and Baker Hughes (NYSE:BHI) have dominated the fracking business for the past several years, and more recently many other players have jumped into the field to get a piece of the action, causing prices to fall in this booming part of the energy sector.

Hydraulic fracturing, or fracking, is a drilling technique that blasts millions of gallons of water and chemicals into the earth to extract natural gas or oil from fractured rocks. The high-pressure drilling technique has been heavily criticized, with activists and environmentalists contending fracking contaminates groundwater.

Halliburton's stock price has fallen 23 percent and Baker Hughes' has declined 41 percent in a two-year period.