In Depth: Columbine, 20 years later

We look back at the Columbine shooting that left 12 students, 1 teacher, and the 2 gunmen dead on April 20, 1999. At the time it seemed unthinkable. But since then, there have been many school shootings, including the ones at Sandy Hook and Parkland, Florida. President Trump and other leaders promised change in the wake of the Parkland shooting, including a proposal to arm teachers with weapons to defend students. We talk to LA School Police Chief Steve Zipperman about security at schools, and what has changed since Columbine 1999.

RELATED: Columbine survivors helping others affected by massacres

In the next segment, we hear from Sue Klebold, mother of one of the Columbine shooters, Dylan Klebold. She speaks at a TedMed Conference on the role of mental illness in school shootings. Also, Psychiatrist Carole Lieberman joins Chief Zipperman to talk about the signs to look for regarding mental illness in young students.

RELATED: Columbine survivors send kids to schools altered by attack

Then, teacher E'bow Morgan of TEACH Academy of Technologies talks about keeping students safe in the classroom. His school uses several strategies, like reaching out to families, and taking phones away from kids to keep them off of social media during class instruction time.