A gunman murdered a woman decorating a tiny church hall for a children's Christmas party and killed two men elsewhere in a rural township before being shot dead in a gun battle with state troopers.
Three troopers in patrol cars were injured in a pursuit that began after the gunman, driving a pick-up truck, fired at them, police said.
One trooper injured a wrist and was then hit in the chest but was saved by his bulletproof vest.
The gunman was killed during a final exchange of gunfire after ramming his truck head-on into another police cruiser, authorities said.
The shootings began in Frankstown Township, in central Pennsylvania, at about 9am yesterday, and investigators were processing five crime scenes within about a 1.5-mile radius.
Troopers were responding to an emergency services call of a shooting in the township when they heard calls reporting at least one other shooting elsewhere, state police said. "It's going to take us some time to put this all together ... and know exactly what occurred," said Lt Col George Bivens, the deputy state police commissioner.
Authorities did not release the names of the victims or the gunman, but said the man lived in Blair County.
State police said they were still trying to piece together a timeline and motive. The gunman and the victims were not related, though the victims may have been, at least distantly, Blair County district attorney Rich Consiglio said.
Lt Col Bivens said investigators did not know if the victims were picked at random.
Besides the trooper wounded twice, a second trooper was injured by glass fragments in his eye and bullet fragments that hit him in the forehead, Lt Col Bivens said. The third trooper suffered minor injuries from a head-on crash.