AVELLA — Three contract workers were injured, one critically, in a fiery explosion that erupted during well testing at a natural gas-drilling site in western Pennsylvania, authorities said.

The three men were not involved with the testing but were removing water caused by melted snow at the Chesapeake Appalachia site near Avella, the company said.

The flash fire erupted at 6:15 p.m. Wednesday and involved five 500-barrel storage tanks before it was extinguished at 9:30 p.m., the company said in a release. No major environmental damage was immediately reported.

Richard Lancaster, 50, of Sardis, Ohio, remained in critical condition and Russell Schoolcraft, 48, of Clover, W.Va., in serious condition at UPMC-Mercy in Pittsburgh, hospital spokeswoman Karissa Millick said Thursday.

Frank Lancaster, 36, of New Martinsville, W.V., remained at West Penn Hospital, where his condition was not released at the request of family.

Chesapeake Appalachia, a subsidiary of Chesapeake Energy Corp., said three wells on the pad were closed after the explosion and will remain closed until an investigation is finished, the company said.

Investigators with the state Department of Environmental Protection were also at the site.