A gunman has shot and killed three girls in a small Amish school in the eastern U.S. state of Pennsylvania.

Police say seven other people were injured Monday morning, some seriously, before the gunman took his own life. A police official described the crime scene as "horrendous." He said the gunman walked into the school, sent the boys and some females outside, then tied up and shot the girls.

The shooting rocked the usually tranquil Amish community, a mainly rural minority Christian group which stresses humility and rejects many of the trappings of modern life. Police say the gunman was not Amish.

The police official said the gunman had left what were described as "rambling suicide notes" in which he said he was seeking revenge for something that happened 20 years ago.

American schools have been jolted over the past week by gun-related incidents. On Friday, a school principal in the central northern state of Wisconsin was shot and killed. A student has been charged with his murder.

Two days before that, a drifter in Colorado took six female high school students hostage. He molested the girls, then shot and killed one of them, before killing himself.

The shooting in Colorado happened in a town not far from the site of the 1999 shooting incident at Columbine High School that killed 15 people.

Some information for this report was provided by AFP, AP and Reuters.