A milk-truck driver carrying three guns and a childhood grudge stormed a one-room Amish schoolhouse, sent the boys and adults outside, barricaded the doors with wood planks, and then opened fire on a dozen girls, killing three people before committing suicide.
At least seven other victims were critically wounded, authorities said.
It was America's third deadly school shooting in less than a week, and it sent shock waves through Lancaster County's bucolic Amish country, a picturesque landscape of horse-drawn buggies, green pastures and neat-as-a-pin farms, where violent crime is virtually nonexistent.
Most of the victims had been shot execution-style at point-blank range after being lined up along the chalkboard, their feet bound with wire and plastic ties, authorities said.
Two young students were killed, along with a female teacher's aide who was slightly older than the students, state police Commissioner Jeffrey B. Miller said.
The gunman, who police identified as Charles Carl Roberts IV, a 32-year-old truck driver from the nearby town of Bart, was bent on killing young girls as a way of "acting out in revenge for something that happened 20 years ago" when he was a boy, Miller said.
Miller refused to say what that long-ago hurt was.
Roberts was not Amish and appeared to have nothing against the Amish community, Miller said. Instead, Miller said, he apparently picked the school because it was close by, there were girls there, and it had little or no security.
Miller said investigators were looking into the possibility the attack may have been related to the death of one of Roberts' own children. According to an obituary, Roberts and his wife, Marie, lost a daughter shortly after she was born in 1997.