While the doc­u­ments show that Mr. Lanza readily had access to weapons, a fact that was already known, by them­selves they do not shed light on his motives, said Jack Levin, a pro­fessor of soci­ology and crim­i­nology at North­eastern Uni­ver­sity who has written sev­eral books on mass murders.

But in many school shoot­ings, the killers were often bul­lied or ostra­cized by their class­mates, “and the motive is revenge,” Mr. Levin said in a tele­phone interview.

And Mr. Lanza did have other traits in common with school gunmen, including social iso­la­tion and access to weapons and firearms training, Mr. Levin said. The clip­ping on the Northern Illi­nois shooting, Mr. Levin said, indi­cates that, like some mass mur­derers, he might have been inspired by past shootings.

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