The medical community understands mass shootings differently than the rest of us. While members of the public observe from a distance, first responders lay their hands on wounded bodies. In emergency rooms, surgeons fight to save lives. And long after the media’s attention moves on, medical professionals continue to help victims and communities heal.

Twenty Years After Columbine,” a striking new multimedia project by The New Yorker, examines the role of doctors in responding to mass shootings. The Trace partnered with The New Yorker to build a database of every mass shooting in the two decades since Columbine: 165 incidents in all, which claimed nearly 1,000 lives.

The doctors interviewed for the project describe the shock of responding to exceptional circumstances, the severity of the injuries they encountered, and the challenges of coping with the effects of trauma in their own lives.

Twenty years ago, Dr. Christopher Colwell was working at Denver General Hospital when he was dispatched to the aftermath of the shooting at Columbine High School in nearby Littleton, where a pair of gunmen had killed 13 people and left 21 others injured. He, like many Americans, considers the incident a watershed moment, when mass shootings become recognized as a distinct phenomenon that can strike anywhere, turning unsuspecting communities into sites of horror.

“Columbine was a real turning point where we started to realize, this not only could happen but will happen, and it will happen in all communities,” Colwell told The New Yorker. “This is not an inner-city issue, this is not a somewhere-else issue. This could happen to us.”

More than a decade later but only a few miles away, Dr. Comilla Sasson was working at the University of Colorado Hospital when victims from the Aurora, Colorado, movie theater shooting began to pour in. She said the patients she treated had “the worst injuries I’ve ever seen.” More than six years later, she’s still coping with what she experienced that day: “It affects you in a way you wouldn’t think.”

But for every shooting that lingers in the public consciousness, many more come and go with far less acknowledgement. The Trace and The New Yorker worked together to fill the gaps by compiling a complete listing of mass shootings. Because there is no authoritative definition of mass shootings, our analysis followed standards established in a 2012 federal law on violent crime, which defines “mass killings” as incidents that occur in public places and claim three or more lives. We excluded shootings linked to other crimes like armed robbery, gang violence, or drugs.

We drew on databases created by Mother Jones, the Stanford Geospatial Center, The Washington Post (which also uses data from Mother Jones and criminologist Grant Duwe), the FBI, and Gun Violence Archive to compile the data. For each incident, the date, location, victim count, and circumstances were verified in at least two media sources.

A timeline of these shootings, interwoven with videos of first responders reflecting on their experiences, forms the core of the multimedia experience. Please visit The New Yorker’s website to hear these doctors’ voices and view the complete multimedia project.