Most students who carried out deadly school shootings first displayed threatening or suspicious behavior that went unreported, according to an analysis released Thursday by the U.S. Secret Service.
The report, by the Secret Service's National Threat Assessment Center, is based on the analysis of 41 incidents of "targeted school violence" in the United States from 2008 to 2017.
"These are not sudden, impulsive acts where a student suddenly gets disgruntled,'' Lina Alathari, the center's head, told the Associated Press. "The majority of these incidents are preventable.''
The reports found that in 80% of the shootings, the attackers' behavior was so alarming that it "elicited concern from bystanders regarding the safety of the attacker or those around them."
The report will be used to train school and law enforcement officials on how to better identify potential attackers and how to stop them.
Among the signs that school officials, families and peers need to be aware of are increased anger, an interest in weapons and violence, depression or isolation, self-harm or a sudden change in behavior, the report said.
It found that while most U.S. schools have security features like cameras in place and have adopted lockdown procedures, only 17% of schools have a system in place where students or families can notify authorities of a student in crisis.
Alathari said the report shows that schools may need to think differently about school discipline and intervention.