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.