This Secret Service and U.S. Dept. Of Education investigation into school shootings and assaults found no simple explanation for why these attacks occurred, but they did identify key prevention questions and resources for effective intervention before an attack takes place. They found that incidents of targeted violence at school are rarely sudden, impulsive acts; prior to most incidents, other people knew about the attacker's idea or plan , even though most did not threaten their targets directly.
http://www.ed.gov/offices/OESE/SDFS/preventingattacksreport.pdf