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Jillian Peterson oversees the grimest Excel spreadsheet you can imagine.

She's a criminologist. Her database tries to make sense of mass shootings. It does that,

column by column, breaking down a shooter's biography into neat piles. There are the basics,

name, age, gender. But also what Jillian calls known predictors of violence.

So things like, did you have an interesting guns? What was your family like growing up?

How well did you do in school? Did you have a job? Were you married?

Do you want to know how much a mass shooter weighed? How tall they are? It's all here. Jillian's

database will tell you that 30% of mass shooters had a diagnosis of psychosis, and then only four of

them were women. Jillian visits with shooters and their families, too, relentlessly updating her

work. She's tracked every shooting of four or more people since 1966. It's now up to,

at last cut, I want to say 189 different pieces of information. I thought of Jillian in January,

...