"Included in the new information is the report that a gun safe was found in the bedroom of 20-year-old Adam Lanza, who 
.
took guns belonging to his mother, Nancy, and shot her in her forehead in her bed. 
.
Then he went to the school in Newtown, where he gunned down 20 children and six staff members before killing himself."

http://www.wptv.com/news/national/adam-lanza-documents-sandy-hook-elementary-school-shooter-owned-a-gun-safe-1400-rounds-of-ammo

The final report is actually silent on where the 22 came from.