Investigators were on the scene Monday of an explosion over the weekend at a natural gas well site in West Virginia operated by Antero Resources, that injured at least five people.

Via:

"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 (0800 GMT) 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 LLC ."

Two tanks were ruptured as a result of the explosion. It is believed that a pump that the employees were working on may have ignited vapors in the immediate area, causing the tanks to rupture. Other tanks on the site were damaged, too. It is not clear whether there was a fire that continued to burn, or if the explosion consumed all of the vapor.

Antero Spokesman Al Schopp said five men are at the West Penn Burn Center suffering from varying degrees of burns. OSHA officials identified the contracting companies the men were working for as Nabors Completion and Production Service, C and R Downhole Drilling LLC, and Willowbend Investments Incorporated. The identities of the men and their conditions have not been released.

A neighbor of the well site in recalls the events leading up to the explosions: