Mystery over what triggered high school shooting

An angry online posting from the 17-year-old boy who opened fire at a Nebraska high school, killing an assistant principal before shooting himself dead, offers some clues about why he turned violent.

As authorities work to sort out what may have led to yesterday’s shooting, those who knew Robert Butler Jr. are struggling to reconcile his final actions with their memories of the fun, outgoing student who liked to make jokes and sometimes got into trouble for talking in class.