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(CNN) – Today has been declared a day of mourning at the University of California, Santa Barbara. The campus and others across California will gather later tonight to remember the victims of Friday night's rampage that left six dead, alongside their murderer. Thirteen others were injured.
The killer left behind plenty of evidence. Rantings posted on YouTube and writings from his journal showed that he first started to plan an attack in December 2012, when he bought his first hand gun.
"Many people have fantasies of revenge that never act on it, but in this case, obviously, these were ongoing, vengeful, angry thoughts, and that being aggrandized was an important part of this," says psychiatrist Gail Saltz, author of "Anatomy of a Secret Life: The Psychology of Living a Lie."
"Being viewed, being read was an important part because, as he says in the video, he felt like a nothing and he wanted to go out letting the world know that he was a something," says Saltz.
But could this shooting rampage been prevented?
It's often difficult for law enforcement to know, and "it's even difficult for mental health professionals that the person is seeing to know who it is going to be, because if they don't say they're going to do it, we can't read minds, and we don't always know, and we're not good at predicting which ones go on to commit the really heinous acts," says Saltz.
For more of our interview with psychiatrist Gail Saltz, check out the video above.