There's a 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 Cullen.

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 Amanda Sagoopers up produces.

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.

I want to take a minute here and just say that Dave is 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.

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