NEW : Gunman copied Columbine shooter 's writings in Web postings . Comment: He was left off mission trip for health reasons , center official says . ` He scared a lot of people in the forum throughout the last few months with some of pretty crazy posts but never anything violent . '' He performed Marilyn Manson , Linkin Park songs at concert , co-worker says .
NEW : Gunman copied Columbine shooter 's writings in Web postings . Comment: He was left off mission trip for health reasons , center official says . He performed Marilyn Manson , Linkin Park songs at concert , co-worker says .