From the report of the shootings at Sandy Hook Elementary School:
While the vast majority of persons interviewed had no explanation for the shooter’s actions, a review of electronic evidence or digital media that appeared to belong to the shooter, revealed that the shooter had a preoccupation with mass shootings, in particular the Columbine shootings41 and a strong interest in firearms. For example, there was a spreadsheet with mass murders over the years listing information about each shooting. #sandyhook