A teenager who opened fire at a Nebraska high school on Wednesday, killing an assistant principal before shooting himself dead, left a message on Facebook which could offer clues as to why he turned violent. Robert Butler Jr, 17, also wounded the principal at Millard South High School in Omaha before fleeing the scene and shooting himself in his car about a mile away. Assistant principal Vicki Kaspar, 58, died at Creighton University Medical Centre on Wednesday... (GunPolicy.org)