The U.S. Department of Justice has opened an antitrust investigation into the market for pressure pumping services used in hydraulic fracturing. Pressure pumping is the process of pumping water and other fluids into a well to break up rock formations and increase the well’s oil or gas production. Two of the companies that received civil investigative demands from the DOJ in connection with this probe are Baker Hughes Inc. and Halliburton Co. DOJ’s spokeswoman confirmed the investigation, saying DOJ’s antitrust division is investigating the possibility of anti-competitive practices involving pressure pumping services performed on oil and gas wells.