To be fair and accurate, the link you provided does not say "reading manifestos of prior shooters" as you've included.
The article does state quite clearly that some of them have studied past mass shooters, but does not indicate the exact means by which that's done. Here's what it actually says:
"The data show that many individuals who engage in mass shootings study past mass shooters — one in five (21.6%) studied other mass shooters, and many are radicalized online. The researchers recommended media literacy education as a means of helping people critically consume information and counter extremist propaganda that facilitates violence."
Those bulleted points in context:
The research team cautioned that the qualitative data, from five interviews, did not lend themselves to generalization, because each individual’s story is unique. There was no single profile of a person who engaged in a mass shooting, but the interviewed mass shooters shared the following traits:
- Early childhood trauma and exposure to violence.
- An identifiable grievance or crisis point.
- Validation of beliefs — finding inspiration in past shootings by others.
- The means to carry out an attack.
What that link has to say about manifestos:
Warning Signs — Leakage
Nearly half of individuals who engaged in mass shootings (48%) leaked their plans in advance to others, including family members, friends, and colleagues, as well as strangers and law enforcement officers. Legacy tokens, such as manifestos, were left behind by 23.4% of those who committed mass shootings.
It's an interesting read, thanks for sharing the link.