The suspect walked into the Giant Eagle supermarket in South Strabane, near Pittsburgh, and demanded money, police detective Mike Schidlmeier said.
A group of shoppers followed the suspect as he ran through the store and behind the building to his getaway car, the (Washington, Pa.) Observer-Reporter said.
One of the shoppers, Vincent Kelly, 46, was shot as he confronted the suspect at the car, and died of multiple gunshot wounds, Washington County Coroner Tim Warco said.
"The car took off like a bat of out hell," said Washington Councilman Joe manning, one of the shoppers chasing the suspect through the store, adding the store was crowded with customers.
Police are searching for the gunman, the newspaper said.