Sources:
Columbine High School Shooting: Victims & Killers - HISTORY
Columbine High School shootings | History & Facts | Britannica
Sandy Hook Elementary School shooting | Facts & Timeline | Britannica
Sandy Hook school shooting - HISTORY
Virginia Tech Shooting - HISTORY
Virginia Tech shooting | United States history | Britannica
Stoneman Douglas High School shooting - Wikipedia
Columbine High School massacre - Wikipedia
See a Timeline of Gun Control in the United States (thoughtco.com)
https://www.sun-sentinel.com/local/broward/parkland/florida-school-shooting/fl-florida-school-shooting-911-calls-coral-springs-20180313-story.html
https://www.apa.org/pubs/info/reports/gun-violence-prevention
https://time.com/5209901/gun-violence-america-reduction/