The worst school shooting incident in the United States occurred on April 20, 1999, when students Eric Harris, 18, and Dylan Klebold, 17, killed 12 students and a teacher and wounded 23 before killing themselves at Columbine High School in Littleton, Colo.
At least one other school shooting has occurred in Nebraska. That shooting happened in February 1995, when a 13-year-old boy shot Chadron middle school teacher Andy Pope in the chest. Pope survived, returning to the classroom less than a week later.
Among other U.S. school shooting incidents:
* Oct. 2, 2006: Charles C. Roberts IV, 32, shot 11 girls execution-style at an Amish school near Nickel Mines, Pa., killing five of them and wounding seven. He then killed himself.
* March 21, 2005: 16-year-old student Jeffrey Weise killed nine people, including his grandfather and his grandfather's companion, at home, then five fellow students, a teacher and a security guard at Red Lake High School in Red Lake, Minn., before killing himself. Seven students also were wounded.
* March 24, 1998: Two boys, 11 and 13, fired on their Jonesboro, Ark., middle school from nearby woods, killing four girls and a teacher and wounding 10 others. Both boys were convicted of murder and can be held until age 21.
* Dec. 1, 1997: Three students were killed and five wounded at Heath High School in West Paducah, Ky. Michael Carneal, 14, pleaded guilty but mentally ill to a charge of murder and is serving life in prison.
* May 21, 1998: Two teenagers were killed and more than 20 people hurt when a boy opened fire at a high school in Springfield, Ore., after killing his parents. Kip Kinkel, 17, was sentenced to nearly 112 years in prison.
* May 1, 1992: Eric Houston, 20, held dozens hostage at his former high school in Lindhurst, Calif. During the standoff, Houston fatally shot three students and one of his former teachers.