Only on the "CBS This Morning" podcast, "Columbine" author Dave Cullen joins "CBS Evening News" anchor Jeff Glor to discuss what has changed in the 20 years since two students entered a Colorado high school and opened fire, killing 12 students and one teacher. At the time, it was the deadliest shooting at a high school in the United States. As one of the first reporters on the scene, Cullen explains how schools, law enforcement and the media have changed their practices since Columbine. Earlier this year, Cullen published "Parkland: Birth of a Movement." He explains how reactions from both those affected by mass shootings and society as a whole have evolved as these tragedies happen more frequently. Plus, hear why Cullen says our understanding of teen depression was the biggest lesson learned from Columbine.