At least three school girls have been shot dead and seven others injured at a one-room Amish school in Pennsylvania before the gunman, a local milk truck driver, turned the gun on himself.
A police spokesman said the shooting was some class of revenge attack for an unspecified incident that had happened more than 20 years ago.
At a press conference the spokesman said it appeared that the man he wanted to target young girls only. He released all the boys who were attending classes at the school and shot all the girls - three of them fatally.
The gunman, identified as Charles Carl Roberts (32), opened fire at an Amish school in rural Pennsylvania about 60 miles (100 km) west of Philadelphia.
The gunman arrived at the school in the late morning and took hostages, a spokesman told a news conference at the scene in Bart Township.
He began shooting an automatic handgun, and police then charged the schoolhouse. The gunman had tied up the girls, while letting the boys and some others leave, Miller added.
He said the three dead girls appeared to have been two students and a teacher's aide - an older girl. The school teaches students aged about 6 to 13.
He shot the victims "execution style" in the head, police said. "It's a horrendous crime scene," Miller said.
Miller said the motive was still being determined, but "apparently he did make a statement to his wife that he was acting out of revenge... for something that occurred 20 years ago."
The incident was the third school shooting in a week in the United States.
The Pennsylvania shooting occurred in a normally placid, rural community where Amish farmers live simply, shunning modern machines and vehicles including cars, travel by horse and buggy and cultivate their land using old-fashioned traditions.
Tourists visit the area to buy antiques and old-fashioned quilts.
The shooting was a shock to a community that one resident called almost crime-free.
The teacher and some visitors fled and ran to a nearby farm for help, while the gunman ordered all the boys out of the school.
He then began to fire his gun, while state police troopers shouted, "Roy! Put the gun down! Put the gun down, Roy!" the witness told local reporters.
The troopers then fired at the gunman, the witness said.