The cause of the fire that erupted on a Marcellus Shale well site in Washington County Wednesday night, injuring three workers, one of them critically, is still not known.

Seven Pennsylvania Department of Environmental inspectors spent Thursday trying to sort through what went wrong at a Chesapeake Energy well in Avella, Washington County, said Katy Gresh, spokeswoman for DEP's Southwest Regional Office, but have yet to determine the cause.

In a press release, Chesapeake said it, too, still is trying to figure out what went wrong when the fire broke out at 6:15 p.m. Wednesday and burned until 9:30 p.m.

The company also identified the three injured men, who all were working for subcontractors at the well:

• Russell Schoolcraft, 48, of Clover, WV, a roustabout for H&H Oil Field Services, was taken to UPNC Mercy in Pittsburgh.

• Richard Lancaster, 50, of Sardis, Ohio, a truck driver working for BBU Services, also was taken to UMPC Mercy;

• Frank Lancaster, 36, of New Martinsville, WV, also a BBU truck driver, was taken to West Penn Hospital, Bloomfield.

Company and hospital officials Mr. Lancaster is in critical condition and Mr. Schoolcraft is in serious condition at UPMC Mercy Hospital. The company says 36-year-old Frank Lancaster remains at West Penn Hospital, where his condition is not being released at the request of family.

Chesapeake's contractors were conducting a well testing operation when the fire began, but the three men were not involved in that procedure, the company said. They were removing water from the well site that had accumulated from melting snow.

The incident began with a flash fire at one of five, 500-barrel storage tanks, igniting the other four storage tanks, the company said.

Scott Rotruck, vice president of corporate development for Chesapeake Energy, said the men did not have life-threatening injuries but a prepared statement did not elaborate.

Mr. Rotruck said it is still unclear what sparked the fire at one of the tanks. The fire at the first caused four more to catch fire.

Mr. Rotruck said the tanks did not contain fracking fluid used in Marcellus Shale drilling. Three wells on the site had been drilled and workers were flaring off gases, but the tanks were beyond the setback limit of 150 feet from the wells.

The wells have been temporarily shut down during the investigation but are still functional.

The company statement said there is no environmental damage and there was no danger to the public.

Richard Bogo, 72, who lives in the Avella Heights section of Independence Township, said he heard a boom and then saw a fire covering the whole hill on a ridge top a half a mile to a mile away.

He said he heard a boom and then a second boom about a half hour later.