from the Article:
After ... arrest last week, authorities found emails on his cellphone describing a plot to burn down the high school, commit sexual assaults and purchase weapons to murder school officials and students there before killing himself, said Orange County district attorney spokeswoman Farrah Emami.
"I can only at this point tell you, he laid out in sufficient detail plans to purchase guns and murder lots of people," Deputy District Attorney Andrew Katz told The Associated Press.

Maybe the shooting in Colorado, Virginia Tech etc. etc. is not so uncommon after all. Authorities got lucky on this one.