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Four people died on a Pennsylvania highway when a gunman shot dead three people and later was killed in a shootout with police, according to Pennsylvania media, citing authorities.
Three state troopers were injured in the incident in Frankstown Township, about 160km east of Pittsburgh.
Investigators suspect the shooter might have been driving when he opened fire, shooting people for unknown reasons, the Pittsburgh Tribune Review reported, citing an official with the Blair County Emergency Management Agency.
"The Pennsylvania State Police have neutralized the active shooter in Frankstown Township, Blair County. There is no longer a threat to residents and visitors to this area from this individual," the Blair County Emergency Management Agency said on its Facebook page.
The shooting took place with much of the United States still highly sensitive to gun violence one week after a gunman killed 20 school children and six adults at an elementary school in Newtown, Connecticut.
The unidentified highway shooter killed two other men and a woman, the Altoona Mirror reported, citing the prosecutor, Blair County District Attorney Richard Consiglio.
One trooper was hit in his bulletproof vest and another was hit by flying glass when the shooter fired on his car, the Mirror said. The third trooper was injured in a crash involving the suspect, the newspaper reported, citing Consiglio.
None of their injuries was considered life-threatening, Consilgio told the Tribune Review.
Calls to Consiglio's office by Reuters went unanswered. Police said they would hold a news conference soon to release details.
Newtown held a moment of silence on Friday in remembrance of the children and teachers killed last week in Connecticut. The National Rifle Association pro-gun lobby issued a statement in response to the Newtown shooting, urging armed guards at the nation's schools.