The Secret Service released a comprehensive study on Thursday that suggests many attacks at U.S. K-12 schools were preventable.
The study evaluated 41 incidents of "targeted school violence" between 2008 and 2017.
It found most students who committed attacks were victims of bullying, had a history of disciplinary issues and communicated their intent to attack ahead of time.
A majority of the attacks involved firearms, and the attacker usually got the gun from their house or a family member's home.
The most common motive was conflict with classmates. And in at least four incidents, the attacks were inspired by other school shootings, such as the one at Columbine High School in 1999.
Aside from those commonalities, the agency says school attackers don't fit a certain profile.
The Secret Service suggests schools establish targeted programs to identify students who show warning signs, and apply intervention strategies to reduce the risk of violence.