Columbine wasn’t the first. There had been other mass shootings at American schools. One in 1997 killed three students and wounded five others at a high school in West Paducah, Ky. A 1998 massacre at a middle school in Jonesboro, Ark., left five dead and 10 wounded.

But no earlier burst of gun insanity shattered the national psyche like the carnage on April 20, 1999, at Columbine High School in Littleton, Colo., outside Denver. The very name Columbine — rooted in the Latin word for dove, an enduring symbol of peace — was instantly transmogrified into a metaphor for a nation gone haywire in its embrace of devastating weaponry. Twenty years later, the attack remains as vivid as yesterday for many Americans, and not only because of its appalling casualty count: 13 killed and 21 wounded, excluding the heavily armed shooters, teenage students who took their own lives.

Columbine was more than the deadliest assault till then on a high school in the United States. It was a defining horror of the nascent digital age. Much of it unfolded onscreen in real time. Cowering students used cellphones to report what they had seen or heard. The possible impact that violent video games and internet trawling had on adolescent minds came wrenchingly to the forefront of debate.

In line with its mission of examining the past to try making sense of the present, the Retro Report series of video documentaries recalls Littleton’s nightmare on its 20th anniversary to explore what we have learned about school shootings across the years — and have yet to learn.