The flames spread to nearby houses and even shattered windows, following the explosion, media reports said.

Three homes on Rustic Ridge Drive were levelled by the explosion, and about a dozen others were damaged, county officials were quoted as saying by The New York Times during a press conference.

Lanny Conley, the chief of the Plum Borough Police Department, said at a press conference as quoted by the newspaper on Sunday afternoon that four adults and one adolescent had died and been recovered from the scene.

The names and ages of the victims have not been revealed so far.

Out of the three injured people, two have been released from the hospital.

The condition of the third injured person was critical.

Emergency medical workers treated 57 firefighters for minor issues while they responded to the explosion and fire, Chief Imbarlina Steve Imbarlina, the assistant chief for Allegheny County emergency services, was quoted as saying by The New York Times.

“That could have been anything from a strain sprain to heat-related issues,” he said. “They were treated on scene and returned back to duty and re-entered the effort.”

An investigation into the incident is currently going on.

The cause of the explosion is still not known.