Two workers have died and four others were injured in an accident late Tuesday night at a power plant in Shippingport, Beaver County, officials said. The accident happened at Bruce Mansfield Power Plant on Ferry Hill Road. A spokesperson for FirstEnergy confirmed the workers were performing maintenance work in an underground enclosure. The two who died were contract workers from Enerfab Corporation. Pennsylvania State Police said the workers who died were inside the pit while the four who were injured were standing above it. A pipe ruptured and sludge entered the pit, which is between 20 and 50 feet deep. The workers above were then overcome by a toxic gas, believed to be hydrogen sulfide. An elbow joint was being replaced on the pipe, which was 25 feet underground. The pipe was supposed to be disabled. "Safety is always our top priority, so something like this is a very tragic accident," FirstEnergy spokeswoman Stephanie Walton said. "We extend our condolences to the families of those killed in this tragic accident, and our thoughts and prayers are also with those who were injured." Channel 11 asked FirstEnergy whose responsibility it was to de-pressurize the pipe before work began. "FirstEnergy is currently investigating and has not determined the specific details of what happened. There is no threat to public safety or other workers, and the plant continues to operate normally," Walton said. Investigators identified the two men who died as Kevin Bachner, 34, and John Gorchock, 42, both of Pittsburgh. Both were husbands and fathers. They were also members of Boilermaker Local Lodge 154, where we've learned John Gorchock's wife worked as a secretary. Three of the four men who were injured have been identified as 31-year-old Mark Wagner, of Pulaski, 31-year-old Thomas Cantwell, of Crafton, and 43-year-old Michael Gorchock, of Pittsburgh. He is the brother of John Gorchock. FirstEnergy said one of their employees was injured as well. The injured workers were flown to Pittsburgh hospitals. Two of them were last listed in critical condition. Officials said there was no threat to public safety as a result of the accident. Operations at the plant continued as normal Wednesday. The Occupational Safety and Health Administration was called to the plant to investigate and information is expected to be available in a few days.