Two workers died and four others were injured in an accident at a Pittsburgh-area coal-fired power plant late Tuesday.

Kevin Patrick Bachner, 34, and John Michael Gorchock, 42, both of Pittsburgh, died at Bruce Mansfield Power Station after inhaling the toxic gas hydrogen sulfide “in a confined, well type area”, state police said.

The two were “performing maintenance in an underground enclosure” around 11:30 p.m. at the plant,  said company spokesperson Stephanie Walton.

Walton said the two who died were contractors with Enerfab, a Cincinnati-based construction and maintenance company. Three of the four injured workers were contractors, and the other was a FirstEnergy employee. They were all taken to local hospitals. In a release, state police identified three of the injured employees: Mark Wagner, 31, of Pulaski, Pa.; Thomas Cantwell, 31, of Crafton; and Michael Gorchock, 43, of Pittsburgh.

“We’re working closely with the employees and their families and supporting them,” said Scott Anderson, CEO of Enerfab. “We’re deeply saddened by what’s happened there.” Anderson said his company was a contract maintenance company and had worked at FirstEnergy for “many years”.

Hydrogen Sulfide, or H2S, is a “colorless, flammable, extremely hazardous gas with a ‘rotten egg’ smell”, according to the U.S. Occupational, Safety & Health Administration (OSHA). The agency notes that the gas “is heavier than air and can collect in low-lying and enclosed, poorly ventilated areas.”

In a statement, state police said that while performing contract work for Penn Energy at the Bruce Mansfield plant, “the workers removed an elbow joint in a pipe. In doing so, H2S gas was released into the air in the confined space, incapacitating the workers. Bachner and Gorchock were unable to make it out of the well and died as a result. Wagner, Cantwell and Gorchock inhaled the gas but were able to make it out of the well and were transported to area hospitals.”

Pennsylvania state police and OSHA are investigating, and the plant continues to operate. Walton said there was no risk to the public.

Walton did not know when the last fatality at the plant occurred, but OSHA records indicate a worker died in an accident there in 1995.

Bruce Mansfield Power Station is the largest power plant in Pennsylvania, and first came online in 1976, according to the Pennsylvania Public Utility Commission.

Officials from Boilermakers Union Local 154, which represents many of the plant’s 350 employees, didn’t immediately respond to a request for comment.

This story has been updated.