WASHINGTON (AP) -- Most of the students who commit deadly school attacks were bullied, had a history of disciplinary trouble and their behavior concerned others, but wasn't reported.
That's according to a study by the U.S. Secret Service's National Threat Assessment Center of 41 school attacks since the 1999 Columbine High School shooting in Colorado.
In the report obtained by The Associated Press, investigators reviewed instances where a current or former student purposefully used a weapon to kill at least one person.
The research will help train school officials and law enforcement on how to better identify students who may be planning an attack and how to stop them before they strike.
Most attacks happened at high schools. In the attacks studied, 19 people were killed and 79 injured.