The first weeks of school should be filled with jitters, anticipation and fresh starts. Instead, in Evergreen, Colo., they were filled with grief and fear. A 16-year-old opened fire at Evergreen High School, wounding two classmates before turning the gun on himself.

Deadly school shootings in the U.S. are tragically common, but they are not inevitable. The 16-year-old who opened fire on his classmates and ultimately took his own life had been exposed to a toxic online community, including a forum in which graphic violent videos and extremism were celebrated.

Our team of analysts has found that at least two other school shooters in the past year were active in similar online spaces. The teenagers who perpetuated attacks at the Abundant Life Christian School in Wisconsin and Antioch High in Tennessee were both steeped in online spaces that glorify violence and amplify extremist narratives. Meanwhile, the perpetrator of the Annunciation Catholic School shooting in Minnesota used names, symbols and phrases that likely originate in such spaces.

Moreover, these teens even referenced one another as inspirations for their own successive attacks.

Today’s school shooters are often driven by a broader online........

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