There's this sick kind of machinery that cranks into gear whenever a mass shooting takes place.

Dave Cullen is a part of the machinery.

Unfortunately, yeah, I'm the mass murder guy, which is awful, but that's what it is.

He wrote the definitive book about Columbine. It transformed him

into a color commentator on the horror of American gun violence. Joining us now is Dave Cohen.

He spent years as a Denver journalist researching and reporting on the Columbine shootings.

Dave, thank you for joining us tonight in these difficult circumstances. Thanks for having me,

Rachel. I do the TV shows when I'm asked to do it.

You described getting a text from a producer that was like,

another fucking shooting.

Yes, that was actually what I'm Anderson Cooper's producers.

That text that Dave got, a year ago today.

It was his first sign that this machinery was starting up all over again.

This time at Marjorie Stoneman Douglas High School in Parkland, Florida.

17, Julia 5, I have the gunshot victim.

I want to take a minute here and just say that Dave has a funny way of talking about mass shootings.

It's as if he's trying not to linger on the trauma itself. He speaks in these big, bright loops of conversation, leaping away from the awful bits. Sitting across from him, the way he spoke, it felt like a form of self-protection. He spent years processing the murders at Columbine.

Yeah, but my strength was pretty clear, like, if you do this ever again, you might not

live through it. I mean, that's like worse, Kate. I mean, that's probably not going to happen.

But, I mean, that's the thing of, like, depression, you have to be careful.

But Dave couldn't get the Parkland shooting out of his head, especially the survivors who began speaking out about gun violence.

So when an editor called, asked if he'd get back out there one more time.

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