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Pennsylvania police are hunting a man who shot and killed an 18-year-old girl in a "road rage incident".

Bianca Nikol Roberson, a recent school graduate, was driving near West Goshen when her car veered off the road.

Police responded to what they initially thought was a fatal collision, but then discovered that Ms Roberson had been shot in the head.

Investigators believe another driver shot her as she attempted to merge into the same lane as a red pickup truck.

The road on which Ms Roberson was driving narrows to a single lane where it joins another main route.

"Her car started jostling with a red pickup truck - not actually hitting - but both of them trying to merge together," prosecutor Tom Hogan told reporters.

"And then she was shot. The man in the red pickup truck shot her directly in the head."

"This was a senseless and brutal act of violence," he added.

Ms Roberson was pronounced dead at the scene. The case is now being treated as a homicide.

Police have issued a call for help in tracing the pickup truck and the suspect, who is described as a white male aged between 30 and 40 with blonde hair and a medium build.

Mr Hogan said "every cop in three states" is looking for the vehicle.

Ms Roberson's father, Rodney, , telling them she was preparing to attend university.

"We were all excited for her," he told the station. "She was excited. We were just happy for her."

Her brother, Dontae Arburg, said she was "very loved."

"She always smiled. Always wanted to entertain and make you smile," he said.