An explosion over the weekend at a natural gas well site in West Virginia operated by Antero Resources injured at least five people, prompting state and federal investigations, local officials and Antero said on Monday.

A spark triggered a flash explosion and a fire after a problem during the flow back process when drilling fluids are pumped into storage tanks, according to Pat Heaster, director of emergency services in Doddridge County, about 100 miles north of Charleston.

The flow back process is necessary after hydraulic fracturing, or fracking, in which millions of gallons of water and chemicals are pumped deep underground to release natural gas from shale rock. Two storage tanks containing brine and fracking fluid from the well exploded at 4 a.m. EDT on Sunday Antero spokesman Alvyn Schopp said. Five workers were taken to hospital with burns, he said.

"We do not know the ignition source, but we suspect it was a methane explosion," said Schopp, vice president at Antero, an oil and natural gas company controlled by Warburg Pincus.

Officials with the West Virginia Department of Environmental Protection (DEP) and the federal Occupational Safety and Health Administration were on the scene and investigating the incident. Kathy Cosco, a DEP spokeswoman, said a pump that malfunctioned appeared to be involved, though no conclusions had yet been made. A containment system around the well site had kept the tank fluids from flooding into the nearby area, Cosco said.